1/1*3+1/3*5+1/5*7+...+1/(2n-1)(2n+1)
=1/2 (1-1/3+1/3-1/5+.+1/(2n-1)-1/(2n+1))
=1/2 (1-1/(2n+1))
=17/35
1-1/(2n+1)=34/35
1/(2n+1)=1/35
2n+1=35
2n=34
n=17