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