#include <stdio.h>
// 練習問題G
 
int func(int n){
    int a = 1; // a₁
    int b = 2; // a₂
    int c;     // aₙ（n ≥ 3 のときに使う）
 
    if(n == 1) return a;
    if(n == 2) return b;
 
    for(int i = 3; i <= n; i++){
        c = -2 * b + 2 * a;
        a = b;
        b = c;
    }
    return c;
}
 
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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