2乘3分之1+3乘4分之1+4乘5分之1+...+29乘30分之1
=1/2-1/3+1/3-1/4+1/4-1/5+……+1/29-1/30
=1/2-1/30
=7/15
2分之1+4分之1+8分之1+16分之1+32分之1+64分之1
=1-1/2+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