#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;
    		b=c;
    		c=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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