对任意的整数n都有(1/n)*[1/(n+1)]=1/n-1/(n+1)
1/2*1/3+1/3*1/4+1/4*1/5+1/6*1/7+1/7*1/8+1/8*1/9
=1/2-1/3+1/3-1/4+……+1/8-1/9
=1/2-1/9
=7/18
对任意的整数n都有(1/n)*[1/(n+1)]=1/n-1/(n+1)
1/2*1/3+1/3*1/4+1/4*1/5+1/6*1/7+1/7*1/8+1/8*1/9
=1/2-1/3+1/3-1/4+……+1/8-1/9
=1/2-1/9
=7/18