Relating the circle centers with the sides

Geometry Level 5

Let Δ A B C \Delta ABC be a triangle in which A B = 8 AB = 8 and B C = 11 BC = 11 . If the measure of A I O = 9 0 \angle AIO = 90^{\circ} , find the measure of A C AC .

  • The notations I I and O O represent the points incenter and circumcenter respectively.


The answer is 14.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Ahmad Saad
Jun 29, 2017

Produce A I AI to P P such that P I = A I . PI = AI. Now, we obtain that O I OI is the \perp bisector of segment A P AP Thus, O P = A O OP = AO \Rightarrow The point P P lies on A B C \odot ABC . Join P B , P C PB , PC . Let E E and F F denote the midpoints of A C AC and A B AB respectively. Join I F , O F , I E , O E IF , OF , IE , OE . Let A P AP intersect B C BC at Q Q . Trivial angle chasing in Δ A I O \Delta AIO gives that O A I = B / 2 C / 2 \angle OAI = \angle B/2 - \angle C/2 and A O I = A / 2 + C / 2 \angle AOI = \angle A/2+ \angle C/2 \Longrightarrow In cyclic quad. A O I F AOIF A I F = A O C = C . \angle AIF = \angle AOC = \angle C. And, in cyclic quadrilateral A I O E AIOE , A O E = A I E = B . \angle AOE = \angle AIE = \angle B. Now, by the excenter lemma, P B = P C = P I = A I PB = PC = PI = AI Clearly, through ASA congruency rule, Δ P C Q Δ I A E \Delta PCQ \cong \Delta IAE and Δ P C Q Δ I A F \Delta PCQ \cong \Delta IAF \Rightarrow B Q = A F BQ = AF , similarly Q C = A E . QC = AE. Adding, B Q + Q C = A E + A F = > a = b 2 + c 2 = > 2 a = b + c = > b = 2 a c = 2 11 8 = 22 8 = 14 BQ + QC = AE + AF => a = \dfrac{b}{2}+ \dfrac{c}{2} => 2a = b + c => b = 2a - c = 2 * 11 - 8 = 22 - 8 = 14

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...