为什么当N取大值时(比如1000)，两个程序输出的值不同？

Question

#include<stdio.h>
int main(){
    int N ;
    scanf("%d",&N);
    double a =2;
    double b = 1;
    int i = 0;
    double sum = 0;
    for(i=0;i<N;i++){
        sum+=1.0*a/b;
        double t =  a; 
        a = a + b;
        b = t;
    }
    printf("%.2lf",sum);       
}

#include<stdio.h>
int main(){
    int N ;
    scanf("%d",&N);
    int a =2;
    int b = 1;
    int i = 0;
    double sum = 0;
    for(i=0;i<N;i++){
        sum+=1.0*a/b;
        int t=  a; 
        a = a + b;
        b = t;
    }
    printf("%.2lf\n",sum);
	     
}

Answer 1

一个是用int，一个是用double，转换过程中数就变了