$$x={x}^{\prime}\mathrm{cos}\theta -{y}^{\prime}\mathrm{sin}\theta \phantom{\rule{0ex}{0ex}}y={x}^{\prime}\mathrm{sin}\theta +{y}^{\prime}\mathrm{cos}\theta $$

$$\frac{\mathrm{\partial}f}{\mathrm{\partial}{x}^{\prime}}=\frac{\mathrm{\partial}f}{\mathrm{\partial}x}\frac{\mathrm{\partial}x}{\mathrm{\partial}{x}^{\prime}}+\frac{\mathrm{\partial}f}{\mathrm{\partial}y}\frac{\mathrm{\partial}y}{\mathrm{\partial}{x}^{\prime}}$$

I don't understand how it is equal to

$$\frac{\mathrm{\partial}f}{\mathrm{\partial}{x}^{\prime}}=\frac{\mathrm{\partial}f}{\mathrm{\partial}x}\mathrm{cos}\theta +\frac{\mathrm{\partial}f}{\mathrm{\partial}y}\mathrm{sin}\theta $$

Can you explain?