1/1*5+1/3*7+1/5*9+.+1/(2n-1)(2n+3)
=【（1-1/5+1/3-1/7+1/5-1/9+.+1/(2n-1)-1/(2n+3) 】÷4
=【1+1/3-1/（2n+1)-1/(2n+3)】÷4
=1/3-（n+1)/[(2n+1)(2n+3)]