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How to judge the monotonous increase or decrease of an interval?
Draw an image yourself. The interval corresponding to the upward trend is monotonically increasing, while the downward trend is decreasing. Just add 2kπ to the upper and lower bounds of the corresponding interval (k is an integer by default), and you don't have to remember complicated mathematical formulas. As shown in the figure:

I only need to master these three in middle school. I wish you a happy study.