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