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#include <stdio.h>
#include <math.h>
void main(){
int radius;
float perimeter;
radius = 6;
perimeter = 2*M_PI*radius;
printf("Perimeter of the Circle = %f \n",perimeter);
}
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C = Pi * r * 2 (r stands for radius)
Or you could do this:
C = Pi * d (d stands for diameter)