推荐回答（1个）
画出正弦函数
余弦函数和正切函数
图像上递增的线代表在该区间单调递增
递减的代表在该区间上单调递减
（正切函数单调递增，无递减区间）
然后把所得结果放入直角坐标系中
研究在坐标系中各象限上该函数的增减性
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