1/3-3/5+5/7-7/9…+19/21）的和

#include<iostream>
using namespace std;
int main()
{
 double i,j,n;
 double sum,k;
 i=1;
 j=3;
 n=1;
 sum=0,k=0;
 while(i<=19)
 {
  k=(i/j)*n;
     sum=sum+k;
  n=-n;
     i=i+2;
  j=j+2;
 }
 
    cout<<sum<<endl;
 
 return 0;
}