Answer
$\prod_{j=n-1}^{2n-1}\frac{n-j}{n+j+1}$
Work Step by Step
1. Let $j=i-1$, we have $i=j+1$ and when $i=n$, we have $j=n-1$ and when $i=2n$, we have $j=2n-1$
2. We can rewrite the original expression as
$\prod_{j=n-1}^{2n-1}\frac{n-j}{n+j+1}$