当x = 0,y = 1/(k+2)
当y = 0,x = 1/k;
Sk = 1/(2*(k+2)*k) = 1/4*(1/k - 1/(k+2))
S1 + S2 + …… + S2007 = 1/4*（1-1/3 + 1/3 - 1/5……-1/2009) = 502/2009 jiu shi zhe yang