1乘2分之1减2乘3分之1减3乘4分之1减……减9乘10分之1
=1/2-[(1/2-1/3)+(1/3-1/4)+...+(1/9-1/10)]
=1/2 - (1/2-1/10)
=1/10
1乘3分之2加3乘5分之2加5乘7分之2加……加97乘99分之2
=(1-1/3)+(1/3-1/5)+(1/5-1/7)+...+(1/97-1/99)
=1-1/99
=98/99