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In the second-to-last step, how did you choose 1 5 0 ∘ instead of 3 0 ∘ to use as arcsin ( 2 1 ) ?
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You are right I should choose both.
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Is there a reason why 6 ∘ is the answer and not 6 8 ∘ ?
Let ∠ A B C = ∠ A C B = θ . Then ∣ B C ∣ = 2 ∣ A B ∣ cos θ . Let ∠ A E B = 3 8 ° , ∠ B A E = 3 3 ° , ∠ A E C = 1 9 ° . Then ∣ A E ∣ ∣ A B ∣ = sin 1 0 9 ° sin 3 8 ° = 2 sin 1 9 ° , ∣ A E ∣ ∣ A C ∣ = sin ( 1 4 ° + 2 θ ) sin 1 9 ° . ∣ A B ∣ = ∣ A C ∣ ⟹ sin ( 1 4 ° + 2 θ ) = 2 1 . So θ can be 8 ° or 6 8 ° .
@Alak Bhattacharya , I have provided a diagram for you.
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Note that △ A B C is isosceles that is A B = A C . We have ∠ E B A = 1 8 0 ∘ − 3 8 ∘ − 3 3 ∘ = 1 0 9 ∘ , ∠ E A C = 1 8 0 ∘ − 2 θ − 3 3 ∘ and ∠ E C A = 1 8 0 ∘ − ( 1 8 0 ∘ − 2 θ − 3 3 ∘ ) − 1 9 ∘ = 2 θ + 1 4 ∘ . By sine rule ,
⎩ ⎪ ⎨ ⎪ ⎧ A B E A = sin 3 8 ∘ sin 1 0 9 ∘ A C E A = sin 1 9 ∘ sin ( 2 θ + 1 4 ∘ ) . . . ( 1 ) . . . ( 2 )
Since A B = A C ,
sin 1 9 ∘ sin ( 2 θ + 1 4 ∘ ) sin ( 2 θ + 1 4 ∘ ) sin ( 2 θ + 1 4 ∘ ) ⟹ 2 θ + 1 4 θ = sin 3 8 ∘ sin 1 0 9 ∘ = 2 cos 1 9 ∘ sin 7 1 ∘ = 2 cos 1 9 ∘ cos 1 9 ∘ = 2 1 = 3 0 ∘ or 1 5 0 ∘ = 6 ∘ or 6 8 ∘ Note that sin ( 1 8 0 ∘ − ϕ ) = sin ϕ and sin ( 2 ϕ ) = 2 sin ϕ cos ϕ also sin ϕ = cos ( 9 0 ∘ − ϕ )