1/2+1/4+1/8+1/16+1/32+1/64
=1/2+1/2-1/4+1/4-1/8+1/8-1/16+1/16-1/32+1/32-1/64
=1-1/64
=63/64
1/1*2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-1/7
=6/7