Max Min Values
1. **Problem:** Find the maximum and minimum values of the function $$f(x,y) = x^2 - xy + y^2 - 2x + y.$$
2. **Step 1: Find the critical points** by setting the partial derivatives equal to zero.
\[\frac{\partial f}{\partial x} = 2x - y - 2 = 0,\quad \frac{\partial f}{\partial y} = -x + 2y + 1 = 0.\]
3. **Step 2: Solve the system of equations**:
From the first equation:
$$2x - y = 2 \implies y = 2x - 2.$$
Substitute into the second equation:
$$-x + 2(2x - 2) + 1 = 0 \implies -x + 4x - 4 + 1 = 0 \implies 3x - 3 = 0 \implies x = 1.$$
Then,
$$y = 2(1) - 2 = 0.$$
So the critical point is at $(1,0)$.
4. **Step 3: Classify the critical point using the second derivative test.**
Calculate second derivatives:
$$f_{xx} = 2, \quad f_{yy} = 2, \quad f_{xy} = -1.$$
Calculate the determinant of the Hessian matrix:
$$D = f_{xx} f_{yy} - (f_{xy})^2 = (2)(2) - (-1)^2 = 4 - 1 = 3 > 0,$$
and since $f_{xx} = 2 > 0$, the critical point is a local minimum.
5. **Step 4: Find the minimum value** by evaluating the function at $(1,0)$:
$$f(1,0) = 1^2 - (1)(0) + 0^2 - 2(1) + 0 = 1 - 0 + 0 - 2 + 0 = -1.$$
6. **Step 5: Check for maximum values.** Since the quadratic form is positive definite around the minimum and there are no boundaries given, the function has no maximum (it tends to infinity as $x,y \to \infty$).
**Final answer:** The function has a local minimum value of $-1$ at the point $(1,0)$ and no maximum.