Let Δ A B C be a triangle in which A B = 8 and B C = 1 1 . If the measure of ∠ A I O = 9 0 ∘ , find the measure of A C .
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Produce A I to P such that P I = A I . Now, we obtain that O I is the ⊥ bisector of segment A P Thus, O P = A O ⇒ The point P lies on ⊙ A B C . Join P B , P C . Let E and F denote the midpoints of A C and A B respectively. Join I F , O F , I E , O E . Let A P intersect B C at Q . Trivial angle chasing in Δ A I O gives that ∠ O A I = ∠ B / 2 − ∠ C / 2 and ∠ A O I = ∠ A / 2 + ∠ C / 2 ⟹ In cyclic quad. A O I F ∠ A I F = ∠ A O C = ∠ C . And, in cyclic quadrilateral A I O E , ∠ A O E = ∠ A I E = ∠ B . Now, by the excenter lemma, P B = P C = P I = A I Clearly, through ASA congruency rule, Δ P C Q ≅ Δ I A E and Δ P C Q ≅ Δ I A F ⇒ B Q = A F , similarly Q C = A E . Adding, B Q + Q C = A E + A F = > a = 2 b + 2 c = > 2 a = b + c = > b = 2 a − c = 2 ∗ 1 1 − 8 = 2 2 − 8 = 1 4
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