- Suppose $ \sim (_1)$ on $\{1, 2, 3, \dots\}$, $ \sim (_2)$ on $\{1, 2, 3, \dots \}$, and ⊥ . Let $ = + $. Find $( = )$ under each condition.
a) Assume $ = _1 = _2$.
b) Assume $_1 \neq _2$. After setting up your summation, identify $, $ and $$ in a finite geometric series using the below formula and evaluate your summation.
$$ + + ⋯ + ^{(−1)} = \frac{(1 − ^)}{ 1 - r}$$
EDIT: So part a) so far I know I should use negative binomial distribution for part a (# of trials until 2 successes). So I could use the formula $\binom{x-1}{r-1}\cdot p ^ r \cdot (1 - p)^{(x - r)}$ with $r = 2$. But I don't know how to continue from that.