#include <stdio.h>
//練習問題G
 
 
int func(int n){
    if(n == 1) return 1;
    if(n == 2) return 2;
 
    int a_n   = 1;  // a1
    int a_n1  = 2;  // a2
    int a_n2;       // a3以降を入れる
 
    for(int i = 3; i <= n; i++){
        a_n2 = -2 * a_n1 + 2 * a_n;  // 漸化式
        a_n = a_n1;
        a_n1 = a_n2;
    }
 
    return a_n1;
}
 
int main(void) {
    int n = 3;
    printf("数列a_nについて、n=%dのときの値は%d\n", n, func(n));
    return 0;
}
 

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