Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Prove it Iran

Let ABCABC be an acute-angled triangle. The altitudes BEBE, CFCF meet at HH. OO is the circumcenter of triangle ABCABC. MM is the midpoint of BCBC. PP is the point on EFEF such that HPHOHP \perp HO. QQ is the point on HAHA such that PQHMPQ \perp HM. Prove that QA=3QHQA = 3QH.

Solution

Let (O)(O) be the circumcircle of triangle ABCABC. AKAK is the diameter of (O)(O). If DD, SS, TT are the reflection points of HH through BCBC, CACA, ABAB, then DD, SS, TT are on (O)(O). It's easily seen that HCKBHCKB is a parallelogram so MM lies on HKHK. Let KHKH intersect (O)(O) again at GG. Lines HPHP intersects the lines BCBC, DKDK, STST, GAGA at RR, XX, YY, ZZ respectively.

Figure 1

Note that EFEF is the median line of triangle HSTHST so HY=2HPHY = 2HP. Because OHYROH \perp YR, according to butterfly theorem for segments YRYR and ZXZX, it is obtained for quadrilateral BCTSBCTS that HR=HY=2HPHR = HY = 2HP. By butterfly theorem for quadrilateral AGDKAGDK it is also obtained that HZ=HX=2HR=2HY=4HPHZ = HX = 2HR = 2HY = 4HP. Note that PQHMPQ \perp HM so PQAGPQ \parallel AG. Noticing the parallel lines, it is deduced that HA=4HQHA = 4HQ, this means QA=3QHQA = 3QH. Hence the proof is done. ■

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