Answer:
[tex]\frac{\partial f}{\partial x} = -70+7\cdot y[/tex], [tex]\frac{\partial f}{\partial y} = 12+7\cdot x[/tex], [tex]\frac{\partial f}{\partial x} (1,-1) = -77[/tex], [tex]\frac{\partial f}{\partial y}(1,-1) = 19[/tex]
Step-by-step explanation:
Let [tex]f(x,y) = 9000-70\cdot x +12\cdot y +7\cdot x \cdot y[/tex], then the first partial derivatives of this multivariate function are, respectively:
[tex]\frac{\partial f}{\partial x} = -70+7\cdot y[/tex] (1)
[tex]\frac{\partial f}{\partial y} = 12+7\cdot x[/tex] (2)
Now we evaluate the partial derivatives at [tex](x,y) = (1, -1)[/tex]:
[tex]\frac{\partial f}{\partial x}(1,-1) = -70+ 7\cdot (-1)[/tex]
[tex]\frac{\partial f}{\partial x} (1,-1) = -77[/tex]
[tex]\frac{\partial f}{\partial y}(1,-1) = 12+7\cdot (1)[/tex]
[tex]\frac{\partial f}{\partial y}(1,-1) = 19[/tex]