Mathematics - Circle Question with Solution | TestHub

Since $a+b=d^2$, the condition becomes $(dl+1{)}^2=b\left(l^2+m^2\right)$. Hence these lines touch $(x-d{)}^2+y^2=b$, with centre $(d,0)$ and radius $\sqrt{b}$. Its tangent length from the origin is $\sqrt{d^2-b}$ (when this length is real).