程序代码：

double foo( unsigned n )
{
    double result = 0;

    double den = 1;
    for( unsigned i=0; i!=n; ++i )
    {
        den *= i+1;
        result += (2*i+1)/den;
    }

    return result;
}

#include <assert.h>

int main( void )
{
    assert( foo(0) == 0 );
    assert( foo(1) == 1 );
    assert( foo(2) == 1+3.0/(1*2) );
    assert( foo(3) == 1+3.0/(1*2)+5.0/(1*2*3) );
}