A is arrangement and c is combination. A is related to order, and c is not related to order. For example, "1 and 2" and "2 and 1" are two different selection methods in A, but they are the same in C and can only be regarded as one selection method.

X stands for sample and p stands for probability.

Ex is the expectation, which is equal to the sum of the product of the value of each x and its probability P(x).