= (1/7*8 + 1/8*9 + 1/9*10) * 70
= (1/7 - 1/8 + 1/8 - 1/9 + 1/9 - 1/10) * 70
= (1/7 - 1/10) * 70
= 10 - 7
= 3
(1/56+1/72+1/90)×70
=[1/(7*8)+1/(8*9)+1/(9*10)]*70
=(1/7-1/8+1/8-1/9+1/9-1/10)*70
=(1/7-1/10)*70
=10-7=3
[1/(7*8)]+[1/(8*9)]+[1/(9*10)]*70=(1/7-1/8+1/8-1/9+1/9-1/10)*70=3/70*70=3
自己看别人的,别老想着看我。就是这么有魅力!!