In the latter case, the basic judgment method is:

1 is to bring x+deltax and x-deltax into the original derivative, and then look at the simplified symbol. deltax & gt0

2. It is more convenient to find the first derivative again, and see whether the value of the double derivative at X is greater than zero or less than zero, so that we can judge whether the first derivative is increasing or decreasing near a zero field where X is minimal, and then we can know the value near it according to the value of the first derivative at X. ..

3, the lazy way, I usually do this, bring in a bad number that is not particularly far away, you can figure it out.

4, it can also assist drawing and facilitate judgment.