第九周

 #include <iostream>
using namespace std;
int main ()
{
  float i,a,b,sum=0.0;
  i=1;
  while(i<=19)
  {
    a=i/(i+1);
    b=-(i+1)/(i+2);
    sum=sum+a+b;
    i=i+2;
  }
  cout<<"sum"<<sum+20.0/21.0<<endl;
  return 0;
}