Don L. answered • 01/30/19

Fifteen years teaching and tutoring basic math skills and algebra

Hi Suc, the equation for the perimeter of a circle is: P_{c} = pi * D, where P is the perimeter and D is the diameter.

For your question, The perimeter of a semicircle is: P_{s} = D + 1/2 * pi * D

Solving this for D gives: D = P_{s} / (1 + 1/2 * pi)

Substituting for P_{s} (10.28) and pi (3.14) gives:

D = 10.28 / (1 + 1/2 * 3.14)

D = 3.997 centimeters, or 4.0 centimeters rounded.

Questions?