Lemma Time!
Discrete Mathematics
Level
4
\[ \large \sum_{r=1}^{19} \frac{ (-1)^{r-1} r }{ {20\choose r} } \]
If the summation above equals to \(\frac mn\) for coprime positive integers \(m\) and \(n\), find the value of \(m+n\).
by
Shubham Garg