Triangle A B C is inscribed in equilateral triangle P Q R . If P C = 3 , B P = C Q = 2 , and angle A C B = 9 0 ∘ , then compute A Q .
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There's a much simpler approach.
Because △ P Q R is an equilateral triangle, ∠ R = ∠ P = ∠ Q = 6 0 ∘ . Labeling x = A R , so A Q = 5 − x . Then by Cosine Rule , we can find the length of B C , A B , A C . By Pythagorean Theorem , B C 2 + A C 2 = A B 2 . Can you carry on from here?
B C = 2 2 + 3 2 − 2 ∗ 3 ∗ 2 ∗ ( 1 / 2 ) s i n ( ∠ P C B ) P B = s i n ( ∠ B P C ) B C ∠ B C P = a s i n ( 0 . 6 5 4 ) → ∠ A C Q = 9 0 − ∠ B C P → ∠ C A Q = 1 2 0 − ∠ A C Q s i n ( ∠ A C Q ) A Q = s i n ( ∠ C A Q ) C Q A Q = 5 8
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Since △ P Q R is an equilateral triangle, we have R Q = P Q = P C + C Q = 3 + 2 = 5 . Let A R = x . By cosine rule, we have:
⎩ ⎪ ⎨ ⎪ ⎧ A B 2 = B R 2 + A R 2 − 2 B R ˙ A R cos 6 0 ∘ = 3 2 + x 2 − 3 x B C 2 = B P 2 + P C 2 − 2 B P ˙ P C cos 6 0 ∘ = 2 2 + 3 2 − 2 ( 3 ) A C 2 = A Q 2 + C Q 2 − 2 A Q ˙ C Q cos 6 0 ∘ = ( 5 − x ) 2 + 2 2 − 2 ( 5 − x ) = x 2 − 3 x + 9 = 7 = x 2 − 8 x + 1 9
By Pythagorean theorem, we have B C 2 + A C 2 = A B 2 :
x 2 − 3 x + 9 ⇒ 5 x x ⇒ A Q = 7 + x 2 − 8 x + 1 9 = 1 7 = 5 1 7 = 5 − x = 5 − 5 1 7 = 5 8