The monotonicity of a function must be understood as monotonicity in each domain, that is, when x 1 x2 is in the domain, it is best to draw a function image on the grass paper every time, so that it is clear at a glance:

Parity, find out their symmetry axis or symmetry center, and grasp their respective characteristics:

Finally, I want to say that the most important thing to learn functions is to combine them into a tree, and there will be various functions behind, so it is best to draw its image when you see a function, which is very helpful to solve problems.