# 点积， 余弦定理，复共轭

a·b=a1b1+a2b2+……+anbn。

a·b=a*b^T，这里的b^T指示矩阵b的转置
A为  m x p 的矩阵，B为 p x n 的矩阵，那么称 m x n 的矩阵C为矩阵AB的乘积，记作 C = AB ，其中矩阵中的第 i 行第 j 列元素可以表示为：

http://baike.baidu.com/item/%E4%BD%99%E5%BC%A6%E5%AE%9A%E7%90%86#2_1

https://en.wikipedia.org/wiki/Complex_conjugate

Geometric representation of z and its conjugate in the complex plane. The complex conjugate is found by reflecting z across the real axis.

In mathematics, the complex conjugate of a complex number is the number with equal real part and imaginary part equal in magnitude but opposite in sign.[1][2] For example, the complex conjugate of 3 + 4i is 3 − 4i.