#include<stdio.h>
#include<stdlib.h>
#include<math.h>
int main(){
    int imax=28, n=3;
    double pi, err, an, bn,M_PI=3.141592653932384626;
    double a3,b3;
    a3=6*(cos(M_PI/6)/2);
    printf("a3は%lf  ",a3);
    b3=6*(tan(M_PI/6)/2);
    printf("b3は%lf\n",b3);
    pi=(a3+b3)/2;
    err = M_PI - pi;
 
    printf("%d %20.18lf\n", n, err); //n=3のときの相対誤差の出力
 
    for(int i=0; i<imax; i++){   
 
        n*=2;
        an=(2*an*bn)/(an+bn) ;
        bn=sqrt(an*bn); 
        pi=(an + bn)/2;
        err = M_PI - pi;                                       
 
        printf("%d %20.18lf\n", n, err); //for文内での相対誤差の出力
    }
    return 0;
}

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