The general formula of 1 / 1x3,1 / 3x5,1 / 5x7,1 / 7x9,1 / 9x11
an=1/[(2n-1)(2n+1)]
2/1x3+2/3X5+2/5x7=2/7x9+2/9x11
calculation..
2/1x3+2/3X5+2/5x7+2/7x9+2/9x11=1-1/3+1/3-1/5.+1/9-1/11=1-1/11=10/11