Chapter 12: Problem 49
Find the first partial derivatives of the following functions. $$F(u, v, w)=\frac{u}{v+w}$$
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Chapter 12: Problem 49
Find the first partial derivatives of the following functions. $$F(u, v, w)=\frac{u}{v+w}$$
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A classical equation of mathematics is Laplace's equation, which arises in both theory and applications. It governs ideal fluid flow, electrostatic potentials, and the steadystate distribution of heat in a conducting medium. In two dimensions, Laplace's equation is $$\frac{\partial^{2} u}{\partial x^{2}}+\frac{\partial^{2} u}{\partial y^{2}}=0.$$ Show that the following functions are harmonic; that is, they satisfy Laplace's equation. $$u(x, y)=e^{-x} \sin y$$
What point on the plane \(x-y+z=2\) is closest to the point (1,1,1)\(?\)
Given the production function \(P=f(K, L)=K^{a} L^{1-a}\) and the budget constraint \(p K+q L=B,\) where \(a, p, q,\) and \(B\) are given, show that \(P\) is maximized when \(K=a B / p\) and \(L=(1-a) B / q\).
Given three distinct noncollinear points \(A, B,\) and \(C\) in the plane, find the point \(P\) in the plane such that the sum of the distances \(|A P|+|B P|+|C P|\) is a minimum. Here is how to proceed with three points, assuming that the triangle formed by the three points has no angle greater than \(2 \pi / 3\left(120^{\circ}\right)\). a. Assume the coordinates of the three given points are \(A\left(x_{1}, y_{1}\right)\) \(\underline{B}\left(x_{2}, y_{2}\right),\) and \(C\left(x_{3}, y_{3}\right) .\) Let \(d_{1}(x, y)\) be the distance between \(A\left(x_{1}, y_{1}\right)\) and a variable point \(P(x, y) .\) Compute the gradient of \(d_{1}\) and show that it is a unit vector pointing along the line between the two points. b. Define \(d_{2}\) and \(d_{3}\) in a similar way and show that \(\nabla d_{2}\) and \(\nabla d_{3}\) are also unit vectors in the direction of the line between the two points. c. The goal is to minimize \(f(x, y)=d_{1}+d_{2}+d_{3} .\) Show that the condition \(f_{x}=f_{y}=0\) implies that \(\nabla d_{1}+\nabla d_{2}+\nabla d_{3}=0\) d. Explain why part (c) implies that the optimal point \(P\) has the property that the three line segments \(A P, B P,\) and \(C P\) all intersect symmetrically in angles of \(2 \pi / 3\) e. What is the optimal solution if one of the angles in the triangle is greater than \(2 \pi / 3\) (just draw a picture)? f. Estimate the Steiner point for the three points (0,0),(0,1) and (2,0).
Imagine a string that is fixed at both ends (for example, a guitar string). When plucked, the string forms a standing wave. The displacement \(u\) of the string varies with position \(x\) and with time \(t .\) Suppose it is given by \(u=f(x, t)=2 \sin (\pi x) \sin (\pi t / 2),\) for \(0 \leq x \leq 1\) and \(t \geq 0\) (see figure). At a fixed point in time, the string forms a wave on [0, 1]. Alternatively, if you focus on a point on the string (fix a value of \(x\) ), that point oscillates up and down in time. a. What is the period of the motion in time? b. Find the rate of change of the displacement with respect to time at a constant position (which is the vertical velocity of a point on the string). c. At a fixed time, what point on the string is moving fastest? d. At a fixed position on the string, when is the string moving fastest? e. Find the rate of change of the displacement with respect to position at a constant time (which is the slope of the string). f. At a fixed time, where is the slope of the string greatest?
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