Multiply the radius by itself, then multiply the result by Pi.
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2*pi*radius = perimeter or circumference of a circle radius = circumference/2*pi
perimeter = pi * diameter, or 2 * pi * radius
you can divide the perimeter(circumference) by pi(3.14) and then once you get your answer you divide that by two giving you the length of the radius of the circle. ex: perimeter = 34pi 34pi/pi 34/2=diameter 16=radius
Radius = 40/2pi cm
To find the perimeter of the curved section of the semi-circle: Perimeter of semicircle = Pi x radius If you also need to the find the perimeter of the straight part of the semicircle (the diameter), it is simply double the radius.