Let $C_1, C_2$ be two circles touching each other externally at the point A and let AB be the diameter of circle $C_1$. Draw a secant $BA_3$ to circle $C_2$, intersecting circle $C_1$ at a point $A_1(\neq A)$, and circle $C_2$ at points $A_2$ and $A_3$. If $BA_1=2$, $BA_2=3$ and $BA_3=4$, then the radii of circles $C_1$ and $C_2$ are respectively.