The first step is to determine the monotonicity of f(x)
已知，函数f(x)在定义域D上有单调性，则：
The monotonicity of function f(x)+a is the same as f(x)
. When a>0, the monotonicity of function a·f(x) is the same as f(x).
When a<0, the monotonicity of function a·f(x) is the same. Monotonicity is the opposite of f(x)
复合函数f(g(x))的单调性：
The monotonicity of the composite function f(g(x)) conforms to the law of "same increase and different decrease"
The second step is to determine the monotonicity of its derivative function f '(x) and the positive and negative of f '(x)
当f(x)单调递增：
如果f'(x)单调递增，f(x)单调变化越快
如果f'(x)单调递减，f(x)单调变化越慢
当f(x)单调递减:与上述相反