1/1*2+1/2*3+1/3*4+1/4*5+.+1/19*20等于多少?

1/1*2+1/2*3+1/3*4+1/4*5+。+1/19*20=1-1/2+1/2-1/3+……+1/19-1/20=1-1/20=19/20