In △ A B C , ∠ A = 1 2 0 ∘ , A B = 1 8 and A C = 2 4 . ∠ A has been trisected by segments A E and A D as shown in the figure above. If A E ⋅ D C B E ⋅ A D = n m , where m and n are coprime positive integers, what is the value of m + n 2 0 1 6 ?
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Sir, do you have any other alternative solutions except for that sir? :D (But this solution is really good. :) )
Consider the Δ A B D where AE is the bisector of ∠ B A D .So, applying bisector theorem
A D A B = E D B E ..............................Equation(1)
Consider Δ A E C where AD is the bisector of ∠ E A C .Again applying the theorem
A E A C = E D C D ..............................Equation(2)
Now dividing equation (1) by equation (2):
A C A B . A D A E = C D B E C D . A E B E . A D = A C A B = 2 4 1 8 = 4 3
By Using the Bisector Theorem in Δ A B D
⟹ B E 1 8 = D E A D ⟹ B E = A D 1 8 D E → ( 1 )
By Using the Bisector Theorem in Δ A E C
⟹ E D A E = D C 2 4 ⟹ D E = 2 4 A E ⋅ D C → ( 2 )
Combining ( 1 ) and ( 2 ) ,
⟹ B E = A D 1 8 ( 2 4 A E ⋅ D C ) ∴ A E ⋅ D C B E ⋅ A D = 2 4 1 8 = 4 3 = n m ⟹ m + n 2 0 1 6 = 2 8 8
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Relevant wiki: Sine Rule (Law of Sines)
Q = A E ⋅ D C B E ⋅ A D = sin ∠ B ⋅ sin ∠ D A C sin ∠ B A E ⋅ sin ∠ C = sin ∠ B ⋅ sin 4 0 ∘ sin 4 0 ∘ ⋅ sin ∠ C = sin ∠ B sin ∠ C = A C A B = 2 4 1 8 = 4 3 Using sine rule Using sine rule again
⟹ m + n 2 0 1 6 = 3 + 4 2 0 1 6 = 2 8 8