Construct triangle XYZ such that xy=8cm and <yxz=<zyx=45°.

Construct locus l1 and l2 a bisector of angle x and y respectively.
Construct locus l3 the midpoint of xy, hence locate p where l1,l2 and l3 meet.
With p at the center construct a circle to touch the sides of the triangle.
Measure the radius of the circle