Df(x_0,y_0)(h_1, h_2)=\frac{\partial f}{\partial x}(x_0,y_0)h_1 + \frac{\partial f}{\partial y}(x_0,y_0)h_2= \\ \langle a, h_1 \rangle \cdot y_0-2\sin \|x_0\|^2\langle x_0, h_1 \rangle \cdot b+\langle a, x_0 \rangle \cdot h_2