Question 21
In the triangle $ABC$, $\angle A>90^\circ$, the incircle touches the side $BC$ and $AC$ at $A_1$ and $B_1$ respectively. The line $A_1B_1$ meets the extension of $BA$ at $X$ such that $\angle CXB=90^\circ$. Suppose $BC^2=AB^2+BC\cdot AC$. Find the size of $\angle A$ in degrees.