Find the critical points of each of the functions and classify them as local maxima, minima, or neither. $f(x, y)=x^2+y^2+6 x-4 y+13$.

Use the maximum-minimum test for quadratic functions to decide whether $(0,0)$ is a maximum, minimum, or saddle point. $f(x, y)=3+2 x^2-x y+y^2$.

The Baraboo, Wisconsin, plant of International Widget Co. uses aluminum, iron, and magnesium to produce high-quality widgets. The quantity of widgets which may be produced using $x$ tons of aluminum, $y$ tons of iron, and $z$ tons of magnesium is $Q(x, y, z)=x y z$. The cost of raw materials is aluminum, $\$ 6$ per ton; iron, $\$ 4$ per ton; and magnesium, $8 per ton. How many tons each of aluminum, iron, and magnesium should be used to manufacture 1000 widgets at the lowest possible cost? [Hint: You want an extreme value for what function? Subject to what constraint?]

Verify the chain rule for the functions and curves. $f(x, y, z)=e^{x y z} ; \sigma(t)=\left(6 t, 3 t^2, t^3\right)$.

Find the equation for the tangent plane to each surface at the indicated point. $x y / z=1 ;(1,1,1)$.

A rectangular mirror with area $A$ square feet is to have trim along the edges. If the trim along the horizontal edges costs $p$ cents per foot and that for the vertical edges costs $q$ cents per foot, find the dimensions which will minimize the total cost.

Find the equation for the tangent plane to each surface at the indicated point. $x y z=1 ;(1,1,1)$.

Suppose that $x=f(t)$ and $y=g(t)$ satisfy the relation. Relate $d x / d t$ and $d y / d t$.

$$(x+y)^3+(x-y)^3=42 e^{-\pi}$$

Verify the chain rule for the functions and curves. $f(x, y, z)=x z+y z+x y ; \boldsymbol{\sigma}(t)=\left(e^t, \cos t, \sin t\right)$.

Use the maximum-minimum test for quadratic functions to decide whether $(0,0)$ is a maximum, minimum, or saddle point. $f(x, y)=y^2$.

Find the equation for the tangent plane to each surface at the indicated point. $y^2-x^2=3 ;(1,2,8)$.

Use the maximum-minimum test for quadratic functions to decide whether $(0,0)$ is a maximum, minimum, or saddle point. $f(x, y)=x^2+y^2-x y$.

Find $\nabla\left(1 / r^3\right)(r \neq 0)$.

Suppose that $x=f(t)$ and $y=g(t)$ satisfy the relation. Relate $d x / d t$ and $d y / d t$. $\cos (x-y)=\frac{1}{2}$