Let be an acute triangle with incentre such that . Suppose that . Let be the intersection of the tangents at to and let be the intersection of the line through parallel to with .
Find in degree measures.
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Let D be a point on B C such that C D = C A . Draw A D , A I , B I & C I .
B D = B C − C D = B C − A C = A I
∠ A D B = ∠ A C D + ∠ D A C = ∠ A C D + 9 0 − 2 ∠ A C D = 9 0 + 2 ∠ A C D = ∠ A I B
In triangles △ A I B and △ B D A , A I = B D , ∠ A I B = ∠ A D B & A B is common to them. Therefore, △ A I B ≅ △ B D A by the S − S − A criterion of congruence.
Hence, ∠ B A I = ∠ A B D = 3 5 ∘ ⇒ ∠ B A C = 2 ∠ B A I = 7 0
∠ A C B = 1 8 0 − 7 0 − 3 5 = 7 5 ∘
Let O be the circumcentre of △ A B C . Draw O B and O C . O B ⊥ B P and O C ⊥ C P . ⇒ B O C P is a cyclic quadrilateral.
△ B P C is isosceles. ⇒ ∠ B C P = 2 1 8 0 − ∠ B P C = 2 ∠ B O C = ∠ B A C .
Observe that, ∠ B Q P = ∠ B A C = ∠ B C P , which points out that B Q C P is a cyclic quadrilateral.
Hence, ∠ A Q C = 1 8 0 − ∠ B Q C = ∠ B P C = 1 8 0 − ∠ B O C = 1 8 0 − 2 ∠ B A C = 1 8 0 − 2 × 7 0 = 40 .