Question 7
Define $n!!$ to be $n(n-2)(n-4)\cdots 3 \cdot 1$ for $n$ odd and $n(n-2)(n-4)\cdots 4 \cdot 2$ for $n$ even. When $\sum_{i=1}^{2009} \frac{(2i-1)!!}{(2i)!!}$ is expressed as a fraction in lowest terms, its denominator is $2^a b$ with $b$ odd. Find $\frac{ab}{10}.$