2. Originally Posted by Raiden_11
Hello,

you only have to use Pythagorean theorem:

With your problem the hypotenuse is (r+R). You'll get:

$(AP)^2 + R^2 = (r+R)^2$

$(AP)^2 = (r+R)^2-R^2$

$(AP)^2 = r^2 + 2rR +R^2-R^2$
$(AP)^2 = r^2 + 2rR$

$|AP| = \sqrt{r^2 + 2rR }$