裂项法：1/1x3+1/2x4+1/3x5+……+1/18x20=1/2×(1-1/3+1/2-1/4+1/3-1/5+……+1/18-1/20)=1/2x(1+1/2-1/19-1/20)=1/2x531/380=531/760
