Find the nth derivative of 1/(ax + b)^1/2

I got ((2n-1)!/2^n+1) * (-a)^n * (ax+b)^(-(1+2n)/2)

but the answer in the back of the book was

(2 * (2^2n)!) / (((-1)^n) * (a^n) * ((2n)!) * (ax+b)^-(n + 0.5))

Not sure whether ive got it wrong or it's just in a different form... never done nth derivatives before though, so thought I'd check