#include<stdio.h>
#include<stdlib.h>
int main(void)
{
int i;
float t,a=2;
float b=1;
float sum=0;

for(i=1;i<=20;i++)
{sum=sum+a/b;
 t=a;
 b=t;
 a=a+b;  
 printf("the tweth number is:%f\n",sum);  //printf语句在for语句里面              
 }
                 
    system("pause");
    return 0;
    }


          把printf语句放在for 语句外面：
#include<stdio.h>
#include<stdlib.h>
int main(void)
{
int i;
float t,a=2;
float b=1;
float sum=0;

for(i=1;i<=20;i++)
{sum=sum+a/b;
 t=a;
 b=t;
 a=a+b;  
                  }
  printf("the tweth number is:%f\n",sum);    //放在for语句外面后有点不一样？？？？？            
               
    system("pause");
    return 0;
    }