is an isosceles obtuse triangle with and . Its incircle, circle has a radius of . Line segment is extended to meet at . Find .
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Since △ A B C is isosceles with ∠ A = 1 3 5 ∘ , then ∠ B = ∠ C = 2 2 . 5 ∘ . Let A E be the altitude of △ A B C which passes through center O of the incircle. Then O B bisects ∠ B and ∠ O B E = ∠ O B A = 1 1 . 2 5 ∘ .
We have B E = tan 1 1 . 2 5 ∘ 1 0 and A B = sin 2 2 . 5 ∘ B E = tan 1 1 . 2 5 ∘ sin 2 2 . 5 ∘ 1 0 ≈ 5 4 . 4 1 5 5 3 0 5 4 .
By sine rule we have sin ∠ A D B A B = sin ∠ A B D A D = sin ∠ B A D B D , then
A D B D ⟹ A D + B D = sin 3 3 . 7 5 ∘ A B ⋅ sin 1 1 . 2 5 ∘ ≈ 1 9 . 1 0 8 1 9 3 2 5 = sin 3 3 . 7 5 ∘ A B ⋅ sin 1 3 5 ∘ ≈ 6 9 . 2 5 7 8 3 3 4 2 ≈ 8 8 . 4