2016 AIME I Problem 6

Attempt Problem 6 of the 2016 AIME I below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2016 AIME I solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

6.

In △ABC\triangle ABC let II be the center of the inscribed circle, and let the bisector of ∠ACB\angle ACB intersect AB‾\overline{AB} at L.L. The line through CC and LL intersects the circumscribed circle of △ABC\triangle ABC at the two points CC and D.D. If LI=2LI = 2 and LD=3,LD = 3, then IC=pq,IC = \frac{p}{q}, where pp and qq are relatively prime positive integers. Find p+q.p + q.

Answer: 13
Concepts:incircle, incenter, and inradiusangle chasingsimilarity
Difficulty rating: 2560
Small Hint:

Show ∠DAI=∠DIA\angle DAI = \angle DIA by angle chasing, so DA=DI=5DA = DI = 5

Big Hint:

Triangles DALDAL and DCADCA share angle DD and have ∠DAL=∠DCA,\angle DAL = \angle DCA, so DC=DA2DLDC = \frac{DA^2}{DL}

Solution:

The incenter II lies on the bisector CL‾,\overline{CL}, between CC and L.L. In triangle ACI,ACI, the exterior angle at II gives ∠DIA=∠IAC+∠ICA.\angle DIA = \angle IAC + \angle ICA. On the other hand, ∠DAB=∠DCB\angle DAB = \angle DCB (both subtend arc DBDB) and ∠DCB=∠ICA\angle DCB = \angle ICA (the bisector), so ∠DAI=∠DAB+∠BAI=∠ICA+∠IAC=∠DIA. \begin{aligned} \angle DAI &= \angle DAB \\ &\quad {}+ \angle BAI \\ &= \angle ICA + \angle IAC \\ &= \angle DIA. \end{aligned} Hence triangle DAIDAI is isosceles with DA=DI=DL+LI=5.DA = DI = DL + LI = 5.

Triangles DALDAL and DCADCA have a common angle at D,D, and ∠DAL=∠DAB=∠DCB\angle DAL = \angle DAB = \angle DCB =∠DCA,= \angle DCA, so they are similar. Therefore DADC=DLDA,\frac{DA}{DC} = \frac{DL}{DA}, giving DC=DA2DL=253.DC = \frac{DA^2}{DL} = \frac{25}{3}.

Finally IC=DC−DI=253−5=103,IC = DC - DI = \frac{25}{3} - 5 = \frac{10}{3}, so p+q=10+3=13.p + q = 10 + 3 = 13.

Problem 5#5
Full Exam

Problem 6 in Other Years