#include <stdio.h>
#include <math.h> // pow関数を使用するため
 
double x(int n) {
    double sum = 1.0; // 最初の項は1
    int i;
    for (i = 1; i < n; i++) { // forループ内の変数宣言を外へ
        sum += pow(-1, i) / (2 * i + 1); // (-1)^i / (2i + 1) の計算
    }
    return 4 * sum; // 4倍して結果を返す
}
 
int main() {
    int n1 = 3, n2 = 10;
    printf("x(%d) = %lf\n", n1, x(n1));
    printf("x(%d) = %lf\n", n2, x(n2));
    return 0;
}
 

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