- How many
coin flipsare expected for the first tail?- $\displaystyle X$ is number of
coin flipsbefore first tail. What is $\displaystyle X$'s dispersion, $\displaystyle D(X)$?

- $\displaystyle \sum_{i=0}^\infty \frac{1}{2^i} = 2$
- $\displaystyle D(X)=E(X-E(X))^2=E(X^2)-(E(X))^2$ ... but what is $\displaystyle E(X^2)$? The answer given is $\displaystyle 2$.