#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=2; 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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