Expectation We define the expected value (also called the expectation or the mean) of a random variable X, with PMF px, by E[X] = XxPX (2). IVariance The variance var(X ) of a random variable X is defined by var (X ) = E (X - E[X]) and can be calculated as var(X ) = > (x - E[X]) px(x). It is always nonnegative. Its square root is denoted by ox and is called the standard deviation.\fLet N be the number of independent tosses of a fair coin until two consecutive heads or two consecutive tails. Give pN, E[N], and var(N)