#include <stdio.h>
//練習問題G
 
int func(int n){
    int a1 = 1, a2 = 2, an;  // 初期値
    int i;
 
    if (n == 1) return a1;
    if (n == 2) return a2;
 
    for (i = 3; i <= n; i++) {
        an = -2 * a2 + 2 * a1;  // an = -2*a_{n-1} + 2*a_{n-2}
        a1 = a2;  // 一つ進める
        a2 = an;
    }
 
    return an;
}
 
int main(void) {
    int n = 4;
    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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