1/20+1/30+1/42+……+1/(39x40)原式可化为：=1/4x5 + 1/5x6 + 1/6x7+……+1/39x40=1/4-1/5+1/5-1/6+1/6-1/7+……+1/38-1\39+1/39-1/40=9/40
