for(i=0;i<m;i++) a+=f(i);
1.i=0 a = a + f(0) 调用double f(int n)这个函数,返回s=1.0,a = a + f(0)->a=1.0.
2.i=1 a = a + f(1) 调用double f(int n)这个函数, 返回s=1.0+1.0/1, a = 1.0 +1.0+1.0/1;
3.i=2 a = a + f(2) 调用double f(int n)这个函数, 返回s=1.0+1.0/1+1.0/2, a=1.0+1.0+1.0+1.0/1+1.0+1.0/1+1.0/2;