Chapter 12: Problem 12
Find the domain of the following functions. $$f(x, y)=\cos \left(x^{2}-y^{2}\right)$$
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Chapter 12: Problem 12
Find the domain of the following functions. $$f(x, y)=\cos \left(x^{2}-y^{2}\right)$$
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Use the Second Derivative Test to prove that if \((a, b)\) is a critical point
of \(f\) at which \(f_{x}(a, b)=f_{y}(a, b)=0\) and \(f_{x x}(a, b)<0
Suppose \(P\) is a point in the plane \(a x+b y+c z=d .\) Then the least distance from any point \(Q\) to the plane equals the length of the orthogonal projection of \(\overrightarrow{P Q}\) onto the normal vector \(\mathbf{n}=\langle a, b, c\rangle\) a. Use this information to show that the least distance from \(Q\) to the plane is \(\frac{|\overrightarrow{P Q} \cdot \mathbf{n}|}{|\mathbf{n}|}\) b. Find the least distance from the point (1,2,-4) to the plane \(2 x-y+3 z=1\)
Consider the ellipse \(x^{2}+4 y^{2}=1\) in the \(x y\) -plane. a. If this ellipse is revolved about the \(x\) -axis, what is the equation of the resulting ellipsoid? b. If this ellipse is revolved about the \(y\) -axis, what is the equation of the resulting ellipsoid?
Traveling waves (for example, water waves or electromagnetic waves) exhibit periodic motion in both time and position. In one dimension, some types of wave motion are governed by the one-dimensional wave equation $$\frac{\partial^{2} u}{\partial t^{2}}=c^{2} \frac{\partial^{2} u}{\partial x^{2}},$$ where \(u(x, t)\) is the height or displacement of the wave surface at position \(x\) and time \(t,\) and \(c\) is the constant speed of the wave. Show that the following functions are solutions of the wave equation. $$u(x, t)=\cos (2(x+c t))$$
Let \(f\) be a differentiable function of one or more variables that is positive on its domain. a. Show that \(d(\ln f)=\frac{d f}{f}.\) b. Use part (a) to explain the statement that the absolute change in \(\ln f\) is approximately equal to the relative change in \(f.\) c. Let \(f(x, y)=x y,\) note that \(\ln f=\ln x+\ln y,\) and show that relative changes add; that is, \(d f / f=d x / x+d y / y.\) d. Let \(f(x, y)=x / y,\) note that \(\ln f=\ln x-\ln y,\) and show that relative changes subtract; that is \(d f / f=d x / x-d y / y.\) e. Show that in a product of \(n\) numbers, \(f=x_{1} x_{2} \cdots x_{n},\) the relative change in \(f\) is approximately equal to the sum of the relative changes in the variables.
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