1/4*9+1/9*14+1/14*19+1/19*24……+1/79*84=[(1/4-1/9)+(1/9-1/14)+……+(1/79-1/84)]/5=(1/4-1/84)/5=(20/84)/5=1/211/5*8+1/8*11+1/11*14+……+1/98*101=[(1/5-1/8)+(1/8-1/11)+……+(1/98-1/101)]/3=(1/5-1/101)/...