注意：f（k）是2k个数之和,f（k+1）就是2k+2个数之和,所以f(k+1)=1-1/2+1/3-1/4+……（1/2（k+1）-1）-1/2（k+1）=f（k）+1/2（k+1）-1）-1/2（k+1）=f（k）+1/(2k+1)(2k+2)