#include <stdio.h>
 
int func(int n) {
    int a;
    int b = 2; // a_2 の値 (a_{n-1})
    int c = 1; // a_1 の値 (a_{n-2})
    int i;
 
    if (n == 1) {
        return 1;
    } else if (n == 2) {
        return 2;
    } else {
        // n = 3 のときは 1 回だけループを実行する
        for (i = 2; i <= n; i++) {
            a = -2 * b + 2 * c; // a_n = -2 * a_{n-1} + 2 * a_{n-2}
            c = b;              // 前々項を更新
            b = a;              // 前項を更新
        }
        return a;
    }
}
 
int main(void) {
    int n = 3;
    printf("数列anについて, n=%dのときの値は%d\n", n, func(n));
    return 0;
}
 

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