Show that the lines $AX’,BY’,CZ’$ are concurrent.

circlescontest-mathgeometrytriangles

Let $ABC$ be a triangle and let $P$ be a point so that $AP,BP,CP$ intersect $BC,CA,AB$ at 3 noncollinear points $X,Y,Z$. Let circle $C_1$ pass through $X,Y,Z$. Let $C_1$ intersect the lines $BC,CA,AB$ again at $X',Y',Z'$ (let $X'=X$ if $C_1$ only intersects $AP$ once). Prove that the lines $AX',BY',CZ'$ are concurrent.

I think it would be useful to split the proof into two cases, one where $P$ is in $ABC$ and another where $P$ is outside of $ABC$. One should be able to use Ceva's theorem to conclude the result. First consider the case where $P$ is inside triangle $ABC.$ If $X,Y,Z$ are on the incircle of $ABC$ the result is obvious. I'm not sure how to proceed from here.

Edit: this link has the solution.

Best Answer

Note $BZ\cdot BZ’=BX\cdot BX’$, etc. by power of a point so we’re done by Ceva twice.

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