1、cosA+cosB = 2 cos((A+B)/2)sin((A-B)/2).

2. The meaning of set: set some specified objects together to form a set, and each object is called an element.

3. Connect images with linear functions. So the image of a function only needs to know two points and connect them into a straight line.

4. When the pumping speed f of the pool is constant, the water quantity g in the pool is a linear function of the pumping time t .. Set the original water quantity in the pool. G = S- feet.

5. Description: A method of describing the common attributes of elements in a set, which is written in curly braces to indicate the set, and certain conditions are used to indicate whether some objects belong to this set.