buzzychaoz wrote:
In cyclic quadrilateral $ABCD$, $AB>BC$, $AD>DC$, $I,J$ are the incenters of $\triangle ABC$,$\triangle ADC$ respectively. The circle with diameter $AC$ meets segment $IB$ at $X$, and the extension of $JD$ at $Y$. Prove that if the four points $B,I,J,D$ are concyclic, then $X,Y$ are the reflections of each other across $AC$.
Well this one took me lotta of timeeee, like >3 hrs so like i dieeeeeeeee.
The hardest part of the problem is to know what u are doing, ok so let $E$ the incenter of $\triangle BCD$ and let $G$ the incenter of $\triangle ABD$ then clearly $BIEC, CEJD, JDGA, AGIB$ are cyclic and by radax using that $BIJD$ is cyclic we get that $ABCD$ is bicentric, let $Z$ be the incenter of $ABCD$ then since $\angle BAZ+\angle BCZ=90$ we get $\angle AZC=90+\angle ABC$ and by more angle chasing u get $\angle XAI+\angle XCI=\frac{\angle ABC}{2}=\angle IAZ+\angle ICZ$ and with this u get $X,Z$ isogonal conjugates w.r.t. $\triangle ABC$ and in a similar way u can get $Z,Y$ isogonal conjugates w.r.t. $\triangle ADC$ now using angle bisector theorem $\frac{AX}{XC}=\frac{AZ}{ZC}=\frac{AY}{YC}$ and since $\angle AXC=90=\angle AYC$ u get $\triangle AXC \sim \triangle AYC$ but since they share $AC$ as common side, this similarity turns out to be a congruency, so u get $AX=AY$ and $CX=CY$ which means $X,Y$ symetric w.r.t. $AC$ as desired, thus we are done
P.S. 800th post!? when did that happen...