#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*c-2*b;
    	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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