expected return 的公式不是w乘以expected return就可以了么？未什么还要减去expe return?

问题如下：

The joint probability of returns, for securities A and B, are as follows:

The covariance of the returns between securities A and B is closest to:

选项：

A.

3%²

B.

12%²

C.

24%²

解释：

Expected return on security A= 0.6 x 25% + 0.4 x 20% = 15% + 8% = 23%

Expected return on security B= 0.6 x 30% + 0.4 x 20% = 18% + 8% = 26%

Cov (R_A, R_B) = 0.6 [(25% - 23%)(30% - 26%)] + 0.4 [(20% - 23%)(20% - 26%)] = 0.6 (2% x 4%) + 0.4 (-3% x -6%)= 12%²