If the chord of contact of $\mathrm{P}\left(x_1, y_1\right)$ with respect to the circle $x^2+y^2=a^2$ meets…
If the chord of contact of $\mathrm{P}\left(x_1, y_1\right)$ with respect to the circle $x^2+y^2=a^2$ meets the circle at $\mathrm{A}$ and $\mathrm{B}$; and if $\lfloor \mathrm{AOB}=90^{\circ}$, then $x_1^2+y_1^2=$