There are two ways to find the range of parameters: the first is a method that you are very familiar with and usually effective: separating parameters.

Simple and effective thinking.

But sometimes this method doesn't work, either the formula is too complicated to separate the parameters, or the separated formula can't find the maximum value.

Therefore, the second method is put forward, that is, by discussing the range of parameters, and then seeing whether the formula of parameters is established within this range, if it is established, the parameters can take values within this range, if it is not established, it means that the parameters cannot take values within this range.

For example, in your question, we can do this: ① When m < 0, see if the formula holds. If it holds, m can be less than 0; if it does not, m cannot be less than 0. Similarly, we will discuss the cases of m=0 and > 0, and finally take the intersection as the value range of m.

If we can't judge whether the formula is true or not, we have to continue to narrow the scope of m to discuss.

So the second method is more difficult to master than the first method. I hope it helps you. As you do more questions, you will often encounter such questions, so you must master this method.