I n a t r i a n g l e A B C , i t i s k n o w n t h a t A B = A C . S u p p o s e D i s t h e m i d p o i n t o f A C a n d B D = B C = 2 t h e n a r e a o f t r i a n g l e A B C w i l l b e
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nice solution Pratik I used coordinate geometry letting coordinates be B(00) , c(20) and hence A (1 2t) giving D (1.5 ) leading to a simple equation in t which is the area of the triangle
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I f ∠ C = X , all angles of the isosceles Δ s a r e g i v e n i n s k e t c h . L e t A B = A C = 2 m . A p p l y i n g S i n L a w t o Δ s A D B a n d C D B , S i n ( 1 8 0 − 2 X ) S i n ( 3 X − 1 8 0 ) = B D A D = 2 m = C D A D = S i n ( X ) S i n ( 1 8 0 − 2 X ) ⟹ S i n ( 2 X ) − S i n ( 3 X ) = S i n ( X ) S i n ( 2 X ) . . . . . . . . . . . . . . . . . . . . N o t e : − S i n ( 2 X ) = 2 ∗ S i n ( X ) ∗ C o s ( X ) , S i n ( 3 X ) = 3 ∗ S i n ( X ) − 4 ∗ S i n ( X ) 3 . S u b s t i t u t i n g a n d s o l v i n g t a k i n g c a r e o f n e g a t i v e s i g n w e g e t X = 6 9 . 2 9 5 o . Since D is the midpoint of AC, a r e a Δ A B C = a r e a 2 ∗ Δ B D C = 2 ∗ 2 ∗ S i n ( X ) ∗ 2 ∗ C o s ( X ) = 2 . 6 4 5 8
L e t A B = A C = x ∴ D C = A D = 2 x N o w a c c . t o A p o l l o n i u s t h . : ⟹ B C 2 + A B 2 = 2 ( B D 2 + D C 2 ) ⟹ 4 + x 2 = 2 ( 4 + 2 x 2 ) ⟹ x = 2 2 ⟹ a r ( Δ ) = 4 a 4 b 2 − a 2 [ w h e r e a = 2 , b = x ] ⟹ a r ( Δ ) = 7
how come BD and BC be equal , some logic is missing..... , plse reply...
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it is given in the question
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can u plse make a diagram for that question and send me plse... , iam totally confused by that diagram....
apolanius,, BC^2+AB^2=2 (BD^2+DC^2) 4+(2DC)^2=2 (4+DC^2) 4+4DC^2=8+2DC^2 2DC^2=4 so, DC=2√2 and the rest is easy
ac=sec A triangles abc similar to cbd sec A/2=2/(sec A/2) sec^2A=8 secA= 2 2^1/2=ac therefore altitude is root 7 by hypotenuse thm. area is 1/2 2*root7 answer is root 7 =2.645
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Here is a way to tackle the problem using vectors-
We are given that ∣ ∣ ∣ b ∣ ∣ ∣ = ∣ c ∣ . Also, ∣ B C ∣ = ∣ B D ∣ = 2 .
∣ B D ∣ = 2 ⟹ ∣ ∣ ∣ ∣ 2 c − b ∣ ∣ ∣ ∣ ∣ ∣ ∣ ∣ 2 c − b ∣ ∣ ∣ ∣ 2 ( 2 c − b ) ⋅ ( 2 c − b ) 4 ∣ c ∣ 2 − 2 ∣ ∣ ∣ b ∣ ∣ ∣ ∣ c ∣ cos A − 2 ∣ ∣ ∣ b ∣ ∣ ∣ ∣ c ∣ cos A + ∣ ∣ ∣ b ∣ ∣ ∣ 2 4 5 ∣ c ∣ 2 − ∣ c ∣ 2 cos A cos A = 2 = 4 = 4 ∵ ∣ r ∣ 2 = r ⋅ r = 4 = 4 ∵ ∣ c ∣ = ∣ ∣ ∣ b ∣ ∣ ∣ = 4 5 − ∣ c ∣ 2 4
But, cos A = 2 ∣ c ∣ 2 2 ∣ c ∣ 2 − 4 due to the law of cosines .
So, 1 − ∣ c ∣ 2 2 = 4 5 − ∣ c ∣ 2 4 ⟹ ∣ c ∣ 2 = 8 ⟹ cos A = 4 3 ⟹ sin A = 4 7
Lastly, Area of △ A B C = 2 1 ∣ ∣ ∣ b × c ∣ ∣ ∣ = 2 1 ⋅ 8 ⋅ 4 7 = 7