Полный дифференциал функции z=tgx/y

[latex]dz = \frac{\partial z}{\partial x}\cdot dx+\frac{\partial z}{\partial y}\cdot dy \\ \frac{\partial z}{\partial x}= (z)'_x= \frac{tgx'}{y}=\frac{\frac{1}{cos^2x}}{y}=\frac{1}{y\cdot cos^2x}\\ \frac{\partial z}{\partial y} = tgx \cdot \frac{1}{y}'=-\frac{tgx}{y^2}\\ dz=\frac{1}{y\cdot cos^2x}\cdot dx-\frac{tgx}{y^2}\cdot dy[/latex]