lab-2l+（a-1）的平方=0
ab = 2
a = 1
b = 2
1\ab+1\（a+1）（b+1）+1\(a+2)(b+2)+...+1\(a+2008)(b+2008)
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + …… + 1/2009 - 1/2010
= 1-1/2010
=2009/2010