3分之1加15分之1加35分之1加63分之1加99分之1
=1/(1x3)+1/(3x5)+1/(5x7)+1/(7x9)+1/(9x11)
=1/2(1-1/3+1/3-1/5+1/5-1/7+1/7-1/9+1/9-1/11)
=1/2x(1-1/11)
=1/2x10/11
=5/11