625. Find the derivative of the function $y=x \tan x+\cot x$.
Solution. $y^{\prime}=(x \tan x)^{\prime}+(\cot x)^{\prime}=(x)^{\prime} \tan x+x(\tan x)^{\prime}+$ $+(\cot x)^{\prime}=\tan x+\frac{x}{\cos ^{2} x}-\frac{1}{\sin ^{2} x}$ [II, III, ; (2), (8), (9)]