Question 174
A circle $C$ of radius $r$ is tangent to $L_1$ and $L_2$, two mutually perpendicular lines in the plane. Circles $C_1$ and $C_2$ have radius $s$. Circle $C_1$ is tangent to $L_1$ and externally tangent to the circles $C$ and $C_2$. Circle $C_2$ is tangent to $L_2$ and externally tangent to the circles $C$ and $C_1$. Find $s$ in terms of $r$.