If the perimeter of the semicircle is 10.28mm. What is the semicircles radius?

The formula for a circle's circumference (circle's unique perimeter) is pi * diameter.

However, since we have a semicircle (half of the circle), our formula would 1/2 * pi * diameter, or just pi * radius.

Set up an equation and solve.

10.28 = pi * radius.

r = (10.28) / pi