1-3, - 2-5,3-35, - 4-63, the nth number is____________________ (find the rules)

1-3, - 2-5,3-35, - 4-63, the nth number is____________________ (find the rules)

If the second number is - 2 / 15, it's easy to do
The molecule is 1, 2, 3, 4 The order of
The denominator is: 1 * 3,3 * 5,5 * 7,7 * 9
[(-1)^(n+1)]n/[(2n-1)(2n+1)]