>>586
1.
int sum1toN(int n)
{
    int i, s;

    for(i = 1, s = 0; i <= n; i++){
        s += i;
    }
    return s;
}

2.
#include <stdio.h>
#define PAI 3.14159
void circle(double r, double *enshu, double *menseki);
int main(void){
    double r, enshu, menseki;
    printf("半径を入力 >>");
    scanf("%lf", &r);
    circle(r, &enshu, &menseki);
    printf("円周 = %f 面積 = %f\n"
                , enshu, menseki);
    return 0;
}

void circle(double r, double *enshu, double *menseki)
{
    *enshu = 2.0 * PAI * r;
    *menseki = PAI * r * r;
    return;
}