2 x Pi x r is equal to the circumference of a circle whose radius is equal to r.
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circumference = 2*pi*radius or pi* diameter
Pi r 2. Pi is a numerical constant roughly 3.142. r is the radius of a circle. 2 in this case is the square of the radius. All this adds up to the area of a circle. Area=Pi x (radius squared)
The perimeter of a semicircle with radius R is pi*R + 2*R = R*(pi + 2)
r=d/2 c=Pi*d 2*r=d c=Pi*2*r where c is the circumference r is the radius and d is the diameter
Circumference = 2*pi*radius So the radius = circumference/2*pi