1, if an+ 1/an=q(q is a non-zero constant) or an/an- 1=q(q is a non-zero constant and n≥2 and n∈N_), then {an} is a geometric series.

2. It is best to establish a system when solving the problems such as the angle formed by straight lines on different planes, the included angle between lines and planes, the dihedral angle, the existence problem, the height, surface area and volume of geometry.

3. If the analytic formula of a given function is complicated, it should be simplified first, and then its parity should be judged.

4. Skillfully use definitions to prove monotonicity of functions, find inverse functions and judge parity of functions.

5. Using the sign-preserving property of linear function on interval, the problem of finding the range of a class of parameters can be solved.