#include <stdio.h>
//練習問題G
int func(int n){
    int a, b=2, c=1;
    int i;
 
    if(n == 1){
        return 1;
    }
    else if(n == 2){
        return 2;
    }
    else{
        for(i = 3; i <= n; i++){
            a = -2*b + 2*c;
            c = b;
            b = a;
        }
        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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