A semi circle with diameter 12cm so how to calculate the perimeter

Perimeter = Length of curved surface + Diameter

To get the length of the curved surface, use the formula to get the circumference of a circle and divide the circumference that you get by 2.

Perimeter of a circle is called circumference and it is expressed by the formula:
C (circumference)=2*pi*r, where r is the radius of the circle and pi is the pythagorean constant = 3.14...
You want to determine the perimeter of a semicircle, therefore you must divide the circumference by 2:
C = 2*pi*r/2
C = pi*r
Answer: Semi circle circumference = pi*r