1-1/3+1/6-1/6+1/12-1/12+1/24-1/24+1/48
=1-1/3+1/48
=2/3+1/48
=11/16
=1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7+1/7/8
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1-1/8
=7/8