Let $M$ be an interior point of the triangle $ABC$ such that $AMC = 90^\circ$, $AMB = 150^\circ$, and $BMC = 120^\circ$. The circumcenters of the triangles $AMC$, $AMB$, and $BMC$ are $P$, $Q$, and $R$ respectively. Prove that the area of $\Delta PQR$ is greater than or equal to the area of $\Delta ABC$.
Problem
Source: Bulgaria 1993
Tags: inequalities, geometry, circumcircle, trigonometry
23.04.2006 20:22
I do not think such a point can exist! Because $M$ is interior, angles AMB, BMC and CMA must add up to 360 degrees, so angle CMA is ninety degrees. But no interior point meets this condition. Is there something wrong?
23.04.2006 20:38
Good problem. I'll give a vague couple steps of a solution--try to fill in the gaps. Let D and E be points outside the triangle s.t. $\angle ABD = 90, \angle DAB = 30, \angle CBE = 90, \angle BEC = 30$. It suffices to prove that the area of ABC is at most one fourth of the area of ADEC (why?). Note that by the trig formula for areas, triangles DBE and ABC have the same area. We now see that we might as well have $\angle ABC = 90$, so the problem becomes algebraic; prove that $[ABD] + [CBE] \ge 2 [ABC]$ $\frac{x^2}{\sqrt{3}} + y^2\sqrt{3} \ge 2xy$ which is simply AM-GM. Edit--in response to JBL, I think he meant $\angle AMC = 90$.
23.04.2006 20:54
Whoops, my bad... yeah, that's right $AMC = 90^\circ$, not $ABC$. EDITED.
23.04.2006 21:01
How do you know that $\angle ABD = 90, \angle DAB = 30, etc.$?
23.04.2006 21:34
Karth wrote: How do you know that $\angle ABD = 90, \angle DAB = 30, etc.$? probability1.01 constructed $D$ and $E$ to meet those conditions.
30.07.2021 19:20
solved also here