1/1*2*3+1/2*3*4+……+1/18*19*20裂项相消法= =

问题描述:

1/1*2*3+1/2*3*4+……+1/18*19*20
裂项相消法= =

1/1*2*3+1/2*3*4+……+1/18*19*20=1/2(1/1x2-1/2x3+1/2x3-1/3x4+.1/18x19-1/19x20)=1/2(1/2-1/19x20)=189/760