Finding the radius of a circle inside of a triangle What is the measure of the radius of the circle inscribed in a triangle whose sides measure $8$, $15$ and $17$ units?

I can easily  understand that it is a  right angle triangle  because of the given edges.  but I don't find any  easy formula to find the radius of the circle.
Hint:

use the fact that the area $A$ (of the triangle) is given by: $A=\frac{pr}{2}$ where $p$ is the perimeter and $r$ the incircle radius. This formula can easily be proved ( divide the triangle in three triangle with a common vertex at $O$) and is valid for a convex polygon..
The nswer is $3$, because one formula for the inradius is given by:
$$\frac{\text{area of triangle}}{\text{semi-perimeter of triangle}}$$