Expectation is an average, which can be similarly understood as a weighted average. The probability corresponding to x is its weight, so Ex is the sum of all Xi×Pi. Dx is a kind of variance, that is, the weighted average of X deviation, and the square (Xi-Ex) of each is multiplied by the sum of the corresponding pi. There is another skill formula to remember between Dx and Ex, that is, DX = E (the square of X)-(the square of EX).