Maths Olympiad Prep

Track / Stage 3 / 243 of 260 #243 of 1964

Problem 243

AMC 10/12, early questions
Geometry Difficulty 3.9 Multiple choice

A point (x,y)(x, y) is to be chosen in the coordinate plane so that it is equally distant from the x-axis, the y-axis, and the line x+y=2x+y=2. Then xx is

Pick one

Official solution

Consider the triangle bound by the x-axis, the y-axis, and the line x+y=2x+y=2. The point equidistant from the vertices of this triangle is the incenter, the point of intersection of the angle bisectors and the center of the inscribed circle. Now, remove the coordinate system. Let the origin be OO, the y-intercept of the line be AA, the x-intercept of the line be BB, and the point be PP.

Notice that xx in the diagram is what we are looking for: the distance from the point to the x-axis (OBOB). Also, OP,BP,OP, BP, and APAP are angle bisectors since PP is the incenter. OPCOPDOPC\cong OPD by AASAAS, and PD=OCPD=OC, since PCODPC||OD, so OC=CP=PD=DO=xOC=CP=PD=DO=x. Therefore, since OA=OB=2OA=OB=2, we have DA=CB=2xDA=CB=2-x. Also, CPBFPBCPB\cong FPB and ADPAFPADP\cong AFP by AASAAS, so AF=FB=DA=CB=2xAF=FB=DA=CB=2-x, and AB=42xAB=4-2x. However, we know from the Pythagorean Theorem that AB=22AB=2\sqrt{2}. Therefore, 42x=22    x=22,C4-2x=2\sqrt{2}\implies x=2-\sqrt{2}, \boxed{\text{C}}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.