\[\frac{\left(a;q\right)_{n-k}}{\left(b;q\right)_{n-k}}=\frac{\left(a;q\right)_{%
n}}{\left(b;q\right)_{n}}\frac{\left(q^{1-n}/b;q\right)_{k}}{\left(q^{1-n}/a;q%
\right)_{k}}\left(\frac{b}{a}\right)^{k},\]