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AS Level Mathematics 9709 | Circular Measures 3 | Angle & Triangle Properties | Raja Shafqat — Transcript

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  1. 0:08Assalam-o-Alaikum, beta. In the last
  2. 0:11class, we studied the geometrical
  3. 0:12properties of a circle. Today, we are
  4. 0:14discussing one more small point.
  5. 0:17Obviously, this point is also from
  6. 0:18O-Level math. Clear, beta? We call
  7. 0:21these angle properties. Basic angle
  8. 0:23properties. Now, what are these basic
  9. 0:24angle properties? There are three or
  10. 0:26four properties that we require. Now
  11. 0:28let's check them one by one. The first
  12. 0:30property—all these properties are
  13. 0:32from O-Level math. Well, whenever we
  14. 0:34talk about O-Level math, let's start
  15. 0:36with another property. Suppose this is
  16. 0:38a straight line. Okay? Suppose this is
  17. 0:40a straight line. Okay? And the angle
  18. 0:42given to you on this straight line, the
  19. 0:45name of this angle is angle A. Come on,
  20. 0:47the angle given to you, let's name this
  21. 0:50angle as angle A. Clear? If this is
  22. 0:53angle A, and I'll call this angle B.
  23. 0:56Let's name this angle as angle B. Okay?
  24. 0:59We named one angle as angle A and the
  25. 1:02other as angle B. Okay? Let me explain
  26. 1:04it again. Let me explain it again that
  27. 1:07if we have this angle A. If this angle
  28. 1:10is A, then this will be angle B. So,
  29. 1:12these two angles are called adjacent
  30. 1:14angles. Adjacent. So, what did we learn
  31. 1:17in O-Level math? Adjacent angles on a
  32. 1:19straight line are supplementary.
  33. 1:22Meaning their sum is always what? 180.
  34. 1:24So we learned in O-Level math that
  35. 1:25whatever angles are on a straight line,
  36. 1:27their sum is always what? 180. What do
  37. 1:30we call this in math language? In math
  38. 1:33language, we call it adjacent angles,
  39. 1:37adjacent angles, adjacent angles on a
  40. 1:41straight line. Adjacent angles on a
  41. 1:45straight line. Adjacent angles on a
  42. 1:49straight line are supplementary. This
  43. 1:52is a very basic condition that a child
  44. 1:54learns in very basic math. We call this
  45. 1:57condition that adjacent angles on a
  46. 1:58straight line are always supplementary.
  47. 2:01Supplementary means their sum should
  48. 2:03always be 180. If I tell you this angle
  49. 2:05is 60, then this angle will obviously
  50. 2:07be what? 120. So that the sum of these
  51. 2:10two is what? The sum of these two
  52. 2:12should be 180. The sum of these two
  53. 2:13should be what? 180. So it is a very
  54. 2:15basic condition. We call it adjacent
  55. 2:17angles on a straight line are
  56. 2:18supplementary. Supplementary means
  57. 2:20their sum should always be what? 180.
  58. 2:23Clear? We used to study one more
  59. 2:25property in O-Level math. We used to
  60. 2:28call it vertically opposite angles.
  61. 2:30Vertically opposite angles. Look, we
  62. 2:31used to show this like this too. Then I
  63. 2:33will plot its diagram and show you. We
  64. 2:36studied in O-Level Math that vertically
  65. 2:39opposite angles are equal. Vertically
  66. 2:43opposite angles are equal. Okay, now
  67. 2:47what was this property? Look, it's a
  68. 2:50small property. By the way, this is
  69. 2:52Class 9 Math. Suppose you have a
  70. 2:54straight line here. Okay? A straight
  71. 2:56line. I plotted another line on the
  72. 2:58straight line that is intersecting it.
  73. 3:00If I tell you, hey, this angle is angle
  74. 3:03C. Let me label it for a minute. This
  75. 3:06angle is your angle C. And let me label
  76. 3:08this angle. This angle is your angle D.
  77. 3:11Okay? One I labeled C, one I labeled D.
  78. 3:14And if I label this angle as angle B,
  79. 3:16okay, there you go, I labeled this
  80. 3:18angle. B and I labeled this angle as A.
  81. 3:22So what did we learn in O-Level Math?
  82. 3:24In O-Level Math, we learned that if
  83. 3:26this angle, let's say it's angle C,
  84. 3:28then angle C equals what angle? It is
  85. 3:30D. We used to say in O-Level Math that
  86. 3:33angle C = angle D and we used to say
  87. 3:35angle B = angle A. Let's see a simple
  88. 3:38proof for this for a minute. If this
  89. 3:41angle is 60, then obviously this angle
  90. 3:43on the straight line will be 180-60 =
  91. 3:44120, which we studied in the previous
  92. 3:46question. If this is 60, then this is
  93. 3:48120, so then this is also a straight
  94. 3:50line. If this is 120, then this will
  95. 3:52also be 60. And then this is also a
  96. 3:54straight line. If this is 60, then this
  97. 3:56will be 120. So technically, these two
  98. 3:59angles became? The same. And
  99. 4:00technically, these two angles became?
  100. 4:02The same. So basically, vertically
  101. 4:05opposite angles were derived from
  102. 4:07adjacent angles. Derived from adjacent
  103. 4:11angles. So these two angles will be
  104. 4:13equal, C and D, and these two angles
  105. 4:15will be equal, A and B. So we call
  106. 4:17these angles vertically opposite angles
  107. 4:19. Vertically opposite angles. We used
  108. 4:21to call these X-shape angles in O-Level
  109. 4:23Math too. Meaning these two angles are
  110. 4:25equal and these two angles are equal.
  111. 4:27So we used to call these X-shape angles
  112. 4:30in O-Level Math too. Used to call them
  113. 4:34X-shape angles. So this is also Class 9
  114. 4:37Math, basic adjacent angles on a
  115. 4:39straight line, meaning those angles
  116. 4:41which are on a straight line, both are
  117. 4:43equal angles. Both are next to each
  118. 4:45other. So their sum is always 180. And
  119. 4:47the second property was O-Level math's
  120. 4:49vertically opposite angles. Vertically
  121. 4:51opposite angles. Condition number three
  122. 4:54. Now we will study two things here.
  123. 4:57Condition number three was called
  124. 4:59alternate angles and corresponding
  125. 5:01angles in O-Level math. Alternate and
  126. 5:03corresponding. Okay? We are studying
  127. 5:05two more properties. In math
  128. 5:07terminology, we also call these
  129. 5:10properties angles formed by parallel
  130. 5:13lines. In math, we also call them
  131. 5:17angles formed by parallel lines. Angles
  132. 5:22formed by parallel lines. We also
  133. 5:24studied this in O-Level math. Obviously
  134. 5:26, all these conditions are from O-Level
  135. 5:27math. Angles formed by parallel lines.
  136. 5:31Look at how. Suppose you have this line
  137. 5:34. And this is another line. You see,
  138. 5:38these two lines are parallel. I drew
  139. 5:40these two lines parallel. Okay? I drew
  140. 5:43these two lines parallel. Here, I drew
  141. 5:45both lines parallel. Let's copy these
  142. 5:48two lines here. Wait a minute. Here.
  143. 5:51Let's get some more space. These two
  144. 5:53lines are parallel. Now suppose this
  145. 5:55line is cutting these two lines. Okay?
  146. 5:58Okay? Like this, I randomly drew a line
  147. 6:00that is cutting both these lines. It is
  148. 6:02cutting both these lines. Now look, if
  149. 6:04I tell you this angle is A, then you
  150. 6:05would say this angle is also the same.
  151. 6:07Let's name it A. Let's name this one C.
  152. 6:09So according to the previous condition,
  153. 6:12these two angles are equal. According
  154. 6:15to the previous condition. Clear?
  155. 6:16According to the previous condition. If
  156. 6:18I tell you this angle is D, then you
  157. 6:20would say this angle is also D. Let's
  158. 6:22name it B. Let's name this one D. So
  159. 6:24that means these two angles are equal
  160. 6:26according to the previous condition.
  161. 6:29These two angles are equal according to
  162. 6:31the previous condition. Clear, dear?
  163. 6:33Okay. So these two were equal by
  164. 6:35vertically opposite. These two were
  165. 6:36equal by vertically opposite. If this
  166. 6:38was 60, we know this is also 60. If
  167. 6:40this was 120, we know what this will be
  168. 6:42? This will also be 120. Okay, now I am
  169. 6:44telling you something new. Now suppose
  170. 6:47I am naming this angle E. I named this
  171. 6:50angle E. I named this angle F. Let's
  172. 6:53just mark it randomly. I named this
  173. 6:56angle F. I named this angle G and this
  174. 6:59angle H. Here is G and here is H. G and
  175. 7:02H. Okay, so now we have studied two
  176. 7:05conditions. Now look, the line
  177. 7:07intersecting is intersecting these two
  178. 7:09parallel lines. That is why, what is
  179. 7:11the condition? Angles formed by
  180. 7:13parallel lines. So, A and C were equal.
  181. 7:16B and D were equal. Similarly, E and F
  182. 7:18will be equal. Will G and H also be?
  183. 7:19They will be equal. So now look at the
  184. 7:22first condition; if this is 60, then
  185. 7:24obviously this is also 60, clear kiddo?
  186. 7:27Now we used to study a condition in O
  187. 7:29Level Maths. Look here, the Z shape
  188. 7:31formed here, there it is. So if I say,
  189. 7:34what is angle C? If this is 60, then
  190. 7:36this will also be 60. So we kept saying
  191. 7:39that angle C and E are both equal.
  192. 7:41Because what shape was formed here? A Z
  193. 7:43shape was formed. So we kept calling
  194. 7:45these alternate angles. In O Level
  195. 7:48Maths, we kept calling them alternate
  196. 7:50angles. Alternate angles, alternate
  197. 7:52angles are equal, and in O Level Maths
  198. 7:54we also used to call it this. There was
  199. 7:57a topic in O Level Maths called '
  200. 7:58Bearings'. Bearings, in math they also
  201. 8:01called this 'Z shape angles', 'Z shape
  202. 8:03angles'. So look, what was formed here?
  203. 8:06A Z shape was formed. So if this is C,
  204. 8:08then C and E will both be what? The
  205. 8:10same. What will C and E both be? The
  206. 8:13same. Clear kiddo? I can also read it
  207. 8:15like this. This Z shape is also formed
  208. 8:17this way, that if this is D, then D and
  209. 8:19G will also both be equal. So that
  210. 8:21means angle C and E will both be equal.
  211. 8:24Angle C = Angle E, so then I can also
  212. 8:26say, look, a Z shape is also being
  213. 8:29formed here. Here it is, so angle D and
  214. 8:31angle G will both be what? The same.
  215. 8:33Angle D = Angle G, so in O Level Maths,
  216. 8:36we used to call these two angles
  217. 8:38alternate angles. We used to call them
  218. 8:41Z shape angles. Alternate angles. Look
  219. 8:43again. If this is 60, then math says
  220. 8:45this is also 60. If this is 60, then
  221. 8:47math says this is also 120. This will
  222. 8:49also be 120. So if this is 60, then
  223. 8:51math says this will also be 60. If this
  224. 8:54is 60, then this will also be 60. If
  225. 8:56this is 120, then this will also be 120
  226. 8:59. Clear, kid? So we used to call these
  227. 9:01alternate angles. Alternate, alternate
  228. 9:04Z-shape angles. Alternate angles,
  229. 9:06meaning look, these two lines are
  230. 9:08parallel. This Z-shape angle is formed.
  231. 9:10If you look at it this way, these two
  232. 9:11lines are also parallel. So, these two
  233. 9:13angles will also be what? The same.
  234. 9:15Then we used to use another word.
  235. 9:17Corresponding angles are equal.
  236. 9:18Corresponding angles. We heard another
  237. 9:22word in O-level math; that word was
  238. 9:24corresponding angles. We used to hear
  239. 9:27another term, corresponding angles are
  240. 9:28equal. Just like alternate angles are
  241. 9:30equal. Corresponding angles. For
  242. 9:32corresponding angles, we used a word in
  243. 9:35math. That word, that sign was F-shape
  244. 9:37angles. F-shape. Note one thing. These
  245. 9:40two lines are parallel. Do you see the
  246. 9:42arrow sign? It's parallel. We learned
  247. 9:44that if this is 60, then this is 120.
  248. 9:47Similarly, if this is 60 and 120, then
  249. 9:49this was also 60 and 120. So, will
  250. 9:52angle A and angle E also be...The same?
  251. 9:55Angle, angle, I am repeating it again.
  252. 9:57This is an F-shape angle. So angle A
  253. 9:58and angle E will be equal. And angle D
  254. 10:00and angle H will also be equal. So I
  255. 10:02can say that angle A is equal to angle
  256. 10:05E. And I can also say that angle D is
  257. 10:08equal to angle H. So we used to call
  258. 10:11these angles corresponding angles.
  259. 10:14Corresponding, so we kept saying that
  260. 10:16corresponding angles are equal.
  261. 10:18Corresponding angles are equal. In math
  262. 10:20, we also called them F-shape angles.
  263. 10:22In math, we also used to call them
  264. 10:24F-shape angles. Look again. If this is
  265. 10:2660, then this will also be 60. If this
  266. 10:28is 120, then obviously this will also
  267. 10:30be 120. So this same situation will
  268. 10:32transfer below too. This is 60. If this
  269. 10:35was 60, then this is also 60. If this
  270. 10:38is 60, then this is also 60. If this is
  271. 10:40120, then this is also 120. Clear, kid?
  272. 10:43This is also 120. So we used to call
  273. 10:45these angles corresponding angles and
  274. 10:46alternate angles. Where are
  275. 10:48corresponding and alternate angles
  276. 10:49formed? Between parallel lines. Between
  277. 10:52parallel lines. Then we also used to
  278. 10:54study one more thing. Interior angles
  279. 10:56are supplementary. Then we used to
  280. 10:58study another condition. What was the
  281. 10:59name of that condition? The name of
  282. 11:02that condition was interior angles are
  283. 11:04supplementary. Interior angles are
  284. 11:08supplementary. Supplementary. Well,
  285. 11:12look, now let's see the proof for this
  286. 11:13too. Supplementary means their sum will
  287. 11:16be what? It will be 180. In math, this
  288. 11:19was also called C-shape angles. Or in
  289. 11:22math, this was also called U-shaped
  290. 11:25angles. C-shaped angles or U-shaped
  291. 11:27angles. C-shape or U-shape. We keep
  292. 11:30calling it C-shape or U-shape. Look how
  293. 11:33? Look, these two lines are parallel.
  294. 11:35Two minutes ago, I gave an example that
  295. 11:37if this is 60, then this will also be
  296. 11:3960. If this is 120, then obviously this
  297. 11:41will also be 120. This will also be 60.
  298. 11:44This will also be 60. This will also be
  299. 11:47120. This will also be 120. So you see
  300. 11:49that this 120 and this 60, what is
  301. 11:51their sum? This is a C-shaped angle.
  302. 11:53Look, this is a C-shaped angle. Here it
  303. 11:56is. So that means what will be the sum
  304. 11:58of E and D? 180. So we used to say in
  305. 12:01O-Level math that angle E and angle D,
  306. 12:04angle D and angle E, what will be their
  307. 12:07sum? It will be 180. So what will we
  308. 12:09call these two angles? How are these
  309. 12:10two angles? Supplementary. Meaning this
  310. 12:12is 120. This is 60. Look, what is the
  311. 12:14sum of both? Similarly, I can also say
  312. 12:16what is the sum of C and G? The sum of
  313. 12:19C and G is also 180. So I can also say
  314. 12:22that angle C and angle G, what is the
  315. 12:25sum of both? 180. What is the sum of
  316. 12:28both of these too? 180. Clear, dear? So
  317. 12:31we learned today that this angle and
  318. 12:32this angle are both equal. A and E,
  319. 12:34both these angles are equal. A and E,
  320. 12:36so can we say why they are equal?
  321. 12:38Corresponding. Angle D and angle H are
  322. 12:40also equal. Why can we say that?
  323. 12:42Corresponding. Then look, we learned
  324. 12:44about interior angles, the sum of D and
  325. 12:47E is 180. Then we studied again that
  326. 12:49these two lines are parallel. The sum
  327. 12:51of C and G is 180. So we studied four
  328. 12:53conditions. Number one, what is the
  329. 12:55angle on a straight line always? 180.
  330. 12:58Angle on a straight line, meaning
  331. 13:00adjacent angles are supplementary. Then
  332. 13:02we studied vertically opposite angles.
  333. 13:03C and D are equal. B and A are equal.
  334. 13:06Vertically opposite angles. Then we
  335. 13:08studied alternate angles, Z-shaped
  336. 13:10angles. C and E are both equal. Then we
  337. 13:13learned that these are also alternate
  338. 13:14angles. D and G are both equal. This
  339. 13:17became an alternate angle. Then we
  340. 13:19studied corresponding angles. Angle A
  341. 13:20and E are both equal. Then we learned
  342. 13:23that angle D and H are both equal.
  343. 13:25Angle B and F are both equal. Angle C
  344. 13:28and, sorry, angle B and G are both
  345. 13:31equal. Angle C and angle F are both
  346. 13:33equal. Clear? So what kind of angles
  347. 13:35are these? Corresponding angles. Then
  348. 13:37we studied, what is the sum of D and E?
  349. 13:39180. Because what shape is being formed
  350. 13:41here? C shape. C shape or U shape. Then
  351. 13:44we studied what the sum of C and G
  352. 13:46would be? This will also be 180.
  353. 13:47Because what shape is being formed here
  354. 13:49as well? C shape. Okay? C shape or U
  355. 13:53shape. Let me explain again. This is
  356. 13:55called a C shape or U shape. Here you
  357. 13:57go, C shape or U shape. C or U shape.
  358. 14:01We studied all these conditions. In O
  359. 14:03Level Math. In O Level Math. I am
  360. 14:05telling you one more small property.
  361. 14:07Then we will start further. So all
  362. 14:09these are adjacent angles, vertically
  363. 14:11opposite angles, alternate angles,
  364. 14:13corresponding angles, interior angles.
  365. 14:16So basically, we studied five points
  366. 14:18here. Basically, we studied five points
  367. 14:21here. Look, in the same way. Now there
  368. 14:24are some properties of triangles as
  369. 14:25well. Angle properties of triangles. I
  370. 14:28am telling you a small sixth point.
  371. 14:30Let's read this. Angle properties of
  372. 14:32triangles. Take a look at this once.
  373. 14:34Then God willing, we will proceed
  374. 14:36further. Its name is angle properties
  375. 14:39of triangles. Angle properties of
  376. 14:42triangles. Okay? This is a small
  377. 14:45condition. Angle properties of
  378. 14:47triangles. Now let's see what needs to
  379. 14:48be done in this. First of all, if I
  380. 14:50tell you that, hey, this is a triangle.
  381. 14:52I just randomly sketched a triangle.
  382. 14:54Here you go, I sketched a triangle.
  383. 14:56This is also obviously all O Level Math
  384. 14:58. I sketched a triangle. Math says if
  385. 15:01you see this is your angle q. Note it.
  386. 15:04Name this angle anything. Suppose this
  387. 15:07angle is Q. Suppose this angle is A.
  388. 15:09Suppose this angle is B and this angle
  389. 15:11is C. Come on, here you go. Let's label
  390. 15:13it like this, suppose this is A.
  391. 15:15Suppose this is B. Suppose this is C
  392. 15:16and suppose this is...Q. Let's name it
  393. 15:18D instead of Q. Name it anything. Here
  394. 15:20you go, this is D. So what did we study
  395. 15:22in O Level Math? We studied a property
  396. 15:25in O Level Math that the exterior angle
  397. 15:27of a triangle. We studied in O Level
  398. 15:30Math that the exterior angle of a
  399. 15:32triangle is equal to the sum of
  400. 15:34interior opposite angles. Well, if any
  401. 15:37student has a doubt, I will prove it to
  402. 15:39you. Look how, if I say this is 60. If
  403. 15:42I say this is 60. You will say this
  404. 15:44angle is 120 because of angles on a
  405. 15:45straight line. This is 120. Then answer
  406. 15:48me one thing. So, if I look at this
  407. 15:51triangle, what should the sum of a + b
  408. 15:54+ 120 be? 180. So, when that subtracts
  409. 15:58over there, what do I find the answer
  410. 16:00for a + b is? 60. That means if this is
  411. 16:0360, then look at these two interior
  412. 16:06angles, which angle is this? Exterior,
  413. 16:09and this is its interior angle. The
  414. 16:11angle next to the exterior is called
  415. 16:12the interior angle. These two are
  416. 16:14adjacent angles. So, math says that
  417. 16:17this exterior angle is equal to the
  418. 16:19opposite interior angles. So, what
  419. 16:22property did we learn in O Level Math?
  420. 16:25We learned the property in O Level Math
  421. 16:28that the exterior angle of a triangle
  422. 16:31...we learned in O Level Math. The
  423. 16:36exterior angle of a triangle is equal
  424. 16:41to the sum of the interior opposite
  425. 16:45angles. The exterior angle of a
  426. 16:48triangle is equal to the sum of the
  427. 16:51interior opposite angles. Listen again.
  428. 16:54This is the exterior angle of the
  429. 16:56triangle. The outer angle. For a minute
  430. 16:58, let me assume. This is 60. If this is
  431. 17:0160, this is exterior. Then who is its
  432. 17:03interior? 120. Now we know that the
  433. 17:06angle sum in a triangle is always 180.
  434. 17:10So, if this 120 subtracts over there,
  435. 17:13the answer for a + b is 60. Meaning a +
  436. 17:16b will indirectly be equal to d. So,
  437. 17:20math says that any triangle's exterior
  438. 17:22angle is equal to its opposite interior
  439. 17:25angles. This is exterior, this is
  440. 17:29interior, and the two angles other than
  441. 17:32the interior are called interior
  442. 17:35opposite angles. Interior opposite
  443. 17:38angles. So, there is a property of a
  444. 17:40triangle that the exterior angle of a
  445. 17:41triangle is equal to the interior
  446. 17:43opposite angles. And one thing everyone
  447. 17:46already knows is when we talk about any
  448. 17:48triangle, what is the interior sum of
  449. 17:50the triangle? What is the sum of all
  450. 17:52the angles in a triangle? It is 180. We
  451. 17:54already know that in a quadrilateral
  452. 17:56the sum is 360 and in a triangle the
  453. 17:58sum is 180. We already know this. We
  454. 18:01already know this. So, what do we call
  455. 18:03this? These are the angle properties of
  456. 18:05a triangle. Okay, I am telling you one
  457. 18:07more small property. Okay? We also
  458. 18:10studied this previously in circle
  459. 18:11properties. So, I will repeat it once
  460. 18:14more so that you remember it. Whenever
  461. 18:16we talk about an isosceles triangle in
  462. 18:18math, look, here it is. If I tell you
  463. 18:21the triangle is isosceles. If these two
  464. 18:23sides are equal, then we know that its
  465. 18:25two base angles will also be equal.
  466. 18:27Clear, son? So we used to say that if,
  467. 18:30let's say, I name this triangle ABC,
  468. 18:33then what did we say? In O-Level maths,
  469. 18:36we used to say that angle B and angle C
  470. 18:38will both be the same. We have already
  471. 18:41studied this property. I explained the
  472. 18:43name of this triangle in the circle
  473. 18:45properties. So, what is the name of
  474. 18:47this property? This is an isosceles
  475. 18:49triangle. What is the name of this
  476. 18:52triangle? Isosceles triangle. We have
  477. 18:55already studied this thing. The name of
  478. 18:57the topic was circle properties. I
  479. 18:59explained there too that when a
  480. 19:01triangle is isosceles, its two sides
  481. 19:03are equal. So its base angles will be
  482. 19:05equal. So the most important property
  483. 19:07of an isosceles triangle is that when
  484. 19:09you drop a line in it, what does it do
  485. 19:12to the base? It bisects the base of the
  486. 19:14isosceles triangle. Clear, son? When
  487. 19:18you drop a line in an isosceles
  488. 19:19triangle, what does it always do to the
  489. 19:21base? It bisects it. This is a property
  490. 19:24of an isosceles triangle. Okay? Of an
  491. 19:27isosceles triangle. So I can say that
  492. 19:29in an isosceles triangle, AB and AC are
  493. 19:31both equal. If these two sides are
  494. 19:33equal, then its base angles will also
  495. 19:36be equal. If AB and AC are both equal,
  496. 19:38then maths says that angle B and angle
  497. 19:40C will also both be equal. Angle B and
  498. 19:43angle C will both be equal. We studied
  499. 19:45this property of an isosceles triangle.
  500. 19:47So when you drop a line in an isosceles
  501. 19:49triangle, what does it always do to the
  502. 19:51base? It bisects it. Bisects. And it
  503. 19:54also divides both these angles equally.
  504. 19:57I have already explained this property
  505. 19:59in circles. Meaning, if I tell you that
  506. 20:01this angle is alpha, you will say that
  507. 20:02this is also alpha. This is a property
  508. 20:04of an isosceles triangle. Meaning, this
  509. 20:06is its line of symmetry. This can fold
  510. 20:08onto this, and this can fold onto this.
  511. 20:11Okay, son? Look at another triangle.
  512. 20:13Equilateral triangle. We also studied
  513. 20:15this in O-Level maths. Look at this one
  514. 20:16too, for once. Because why are we
  515. 20:17studying all these things? Because
  516. 20:19these are all required. They will be
  517. 20:20used in this. So the name of this
  518. 20:23triangle is an equilateral triangle.
  519. 20:27Equilateral triangle. Because we have
  520. 20:30already studied the right-angled
  521. 20:31triangle. The name of this point is an
  522. 20:33equilateral triangle. Well, we also
  523. 20:35studied in O-Level maths that any
  524. 20:36triangle whose three sides are equal is
  525. 20:38called an equilateral triangle in maths
  526. 20:40. Okay? What do we call it in maths?
  527. 20:42Let's draw a triangle. Suppose this is
  528. 20:45a triangle and all three sides of this
  529. 20:47triangle are equal. Suppose we name
  530. 20:49this triangle, let's name it. I have
  531. 20:53named the triangle ABC, here it is. I
  532. 20:55have named this triangle A, B, C. We
  533. 20:57have named this triangle A, B and C. So
  534. 21:00if the triangle is equilateral, then
  535. 21:02equilateral means what will its three
  536. 21:05sides be? They will be equal. So what
  537. 21:07did we learn in O-Level Math that when
  538. 21:09we talk about equilateral, an
  539. 21:10equilateral is also called a regular
  540. 21:12polygon. So that means if this is angle
  541. 21:14B, this is angle C, and this is angle A
  542. 21:15, then we know these three angles will
  543. 21:17be equal. So what did we learn in
  544. 21:19O-Level Math that if a triangle is
  545. 21:21equilateral, it means its three sides
  546. 21:23are equal. Meaning all three of its
  547. 21:25sides are equal. So then we learned in
  548. 21:27O-Level Math that its three angles will
  549. 21:29also be equal. Angle A, angle B, angle
  550. 21:32C, and we know we can say that all
  551. 21:35three angles will be the same, and what
  552. 21:37will all three angles be? 60. Because
  553. 21:41what we did was divide the angle sum
  554. 21:43equally. So in two or three questions
  555. 21:45in past papers, you will also find
  556. 21:47equilateral triangles. You will also
  557. 21:49find isosceles triangles and also this
  558. 21:51property that the exterior angle of a
  559. 21:53triangle is equal to the sum of
  560. 21:55interior opposite angles. And we all
  561. 21:58know this fact that in any triangle,
  562. 21:59what is the sum of the angles always?
  563. 22:02The sum of the angles is always 180. We
  564. 22:05all already know this. Clear, kid? So
  565. 22:07we haven't learned anything new yet. So
  566. 22:09this point we studied today is called
  567. 22:11geometrical properties, or let's call
  568. 22:13them angle properties. Angle properties
  569. 22:16. And we studied two things in this
  570. 22:19today. Adjacent, vertically opposite,
  571. 22:22alternate, corresponding, and interior.
  572. 22:26We studied these five properties. Then
  573. 22:28I told you about triangles. The
  574. 22:30triangle property that the exterior
  575. 22:31angle of a triangle is equal to the sum
  576. 22:33of the interior opposite angles. I told
  577. 22:35you about the isosceles property. Sides
  578. 22:37equal, base angles equal. When we drop
  579. 22:39a line, what will it do to the base?
  580. 22:41Bisect. We can also say that an
  581. 22:43equilateral triangle is a pair of three
  582. 22:45equal sides. I can say it that way too.
  583. 22:48These two sides are also equal to each
  584. 22:50other. These two sides are also equal.
  585. 22:52So technically, three pairs have been
  586. 22:54formed. This and this are equal. One,
  587. 22:56this and this equal, two, this and this
  588. 22:58equal, three, so technically
  589. 23:00equilateral means all sides are equal.
  590. 23:03So what happens in an equilateral
  591. 23:04triangle? There are three sides. There
  592. 23:07are three pairs of two equal sides.
  593. 23:09Three pairs of equal sides. Clear, dear
  594. 23:11? Alright, let's start the next point.
  595. 23:14This is mensuration. The mensuration
  596. 23:17related to circles that we studied in O
  597. 23:19Level Maths. That mensuration is also
  598. 23:21required for this topic. There is one
  599. 23:24more point after this. Then finally we
  600. 23:26will start our chapter. Mensuration.
  601. 23:28Let's start from absolute zero.
  602. 23:30Whatever mensuration related to circles
  603. 23:33we studied in O Level Maths, we need
  604. 23:35all of it. Number one, first of all,
  605. 23:38let's start with the circle in the
  606. 23:39circle family. We are starting the
  607. 23:42circle family with the circle itself.
  608. 23:45Now we need a bit of mensuration. What
  609. 23:47did we study in O Level Maths? We
  610. 23:49studied in O Level Maths, obviously,
  611. 23:51that this is a circle. Clear? We
  612. 23:54studied in O Level Maths that this is a
  613. 23:56circle. Alright? In the next class,
  614. 23:58InshaAllah, we will continue from here,
  615. 24:00dear. Okay?

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