A triangle has sides with lengths of 5, 8, and 3. What is the radius of the triangles inscribed circle?
There exists no triangle with the given measurements because the given measurements form a straight line.
If you still want to calculate the are then I have explained below.
And the semi perimeter
Mathematically, since the area comes out to be zero therefore it proves that it is a straight line it is not a triangle, and since there is no triangle so there is no in-circle and thus no in-radius.