1/(1*3)+1/(3*5)+1/(5*7)+……+1/【(2n-1)(2n+1)】
=1/2*(1-1/3)+1/2*(1/3-1/5)+……+1/2*【1/(2n-1)-1/(2n+1)】
=1/2*(1-1/(2n+1))
=17/35
n=17
1/(1*3)+1/(3*5)+1/(5*7)+……+1/【(2n-1)(2n+1)】
=1/2*(1-1/3)+1/2*(1/3-1/5)+……+1/2*【1/(2n-1)-1/(2n+1)】
=1/2*(1-1/(2n+1))
=17/35
n=17