Y by Adventure10, Mango247
Circles
and
meet at
and
. Let
be a point on
. Let
and
meet
again at
and
respectively. The lines
and
meet at
. Show that, as
varies on
,
lies on a fixed circle.

















[asy]
import graph; size(300); real lsf = 0.5; pen dp = linewidth(0.7) + fontsize(10); defaultpen(dp); pen ds = black; pen ttttff = rgb(0.2,0.2,1); pen xdxdff = rgb(0.49,0.49,1); pen ffttww = rgb(1,0.2,0.4); pen ttccqq = rgb(0.2,0.8,0);
draw(circle((2.16,2.22),3.5),ttttff+linewidth(1.2pt)); draw(circle((6.4,2.1),1.86),ttttff+linewidth(1.2pt)); draw((8.13,2.79)--(2.74,-1.23),ffttww+linewidth(2pt)); draw((8.13,2.79)--(0.34,5.21),ffttww+linewidth(2pt)); draw((0.34,5.21)--(5.27,0.62),ffttww+linewidth(2pt)); draw((5.36,3.64)--(2.74,-1.23),ffttww+linewidth(2pt)); draw(circle((5.71,2.12),1.56),ttccqq+linetype("5pt 5pt")); draw((5.36,3.64)--(5.27,0.62),ffttww+linewidth(2pt));
dot((2.16,2.22),ds); label("$S_1$", (1.72,2.12),NE*lsf); dot((6.4,2.1),ds); label("$S_2$", (6.48,2.22),NE*lsf); dot((5.36,3.64),ds); label("$L$", (5.34,3.92),NE*lsf); dot((5.27,0.62),ds); label("$M$", (5.24,0.18),NE*lsf); dot((8.13,2.79),ds); label("$P$", (8.34,2.82),NE*lsf); dot((2.74,-1.23),ds); label("$R$", (2.62,-1.66),NE*lsf); dot((0.34,5.21),ds); label("$Q$", (0.06,5.34),NE*lsf); dot((4.25,1.57),ds); label("$K$", (3.8,1.44),NE*lsf); clip((-4.3,-10.94)--(-4.3,6.3)--(16.18,6.3)--(16.18,-10.94)--cycle); [/asy]