2*pi*radius or pi*diameter
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to find the perimeter of a circle you simply do this (pi x D )
The perimeter of a circle has the formula 2πr. Therefore the length of a semi circle is πr. But the perimeter of a semi circle also includes the diameter which is 2r. Therefore the perimeter of a semi circle = πr + 2r = r(π + 2)
There is no specific formula for a sector of a circle. There is a formula for its angle (at the centre), its perimeter, its area.
The formula for the circumference of a circle is pi * diameter or (2 radius.)
The perimeter is equal to Pi times the diameter.