问一道简便计算！！

1/2+1/6+1/12+1/30+1/42+1/56
＝1/1*2=1/2*3+1/3*4+...+1/7*8
=(1-1/2)+(1/2-1/3)+....+(1/7-1/8)
=1-1/8
=7/8

1-1/2+1/2-1/3+1/3-1/4+1/5-1/6+1/6-1/7+1/7-1/8=33/40

1/1*2+1/2*3+1/3*4+1/5*6+1/6*7+1/7*8
=1-1/2+1/2-1/3+1/3-1/4+1/5-1/6+1/6-1/7+1/7-1/8
=1-1/4+1/5-1/8
=自己算吧

原式=1/1*2+1/2*3+1/3*4+.....+1/7*8
=1-1/2+1/2-1/3+1/3-1/4+....+1/7-1/8
=1-1/8
=7/8