Two tangents $\overline{PA}$ and $\overline{PB}$ are drawn to a circle, where $P$ lies outside the circle, and $A$ and $B$ lie on the circle. The length of $\overline{PA}$ is $4,$ and the circle has a radius of $4.$ Find the length $AB.$
See the following :
Note that AB is just the length of a diagonal of a square with a side of 4 = 4sqrt (2)
+1725 
