Chapter 12: Q34E (page 517)
In Problems \(31 - 34\) find values of m so that the function \(y = m{e^{mx}}\) is a solution of the given differential equation.
\(2y'' + 7y' - 4y = 0\)
Short Answer
The value of \(m\) is \(\frac{1}{2}\) or \( - 4\).
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Chapter 12: Q34E (page 517)
In Problems \(31 - 34\) find values of m so that the function \(y = m{e^{mx}}\) is a solution of the given differential equation.
\(2y'' + 7y' - 4y = 0\)
The value of \(m\) is \(\frac{1}{2}\) or \( - 4\).
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In Example \(7\) we saw that \(y = {\phi _1}(x) = \sqrt {25 - {x^2}} \) and \(y = {\phi _2}(x) = - \sqrt {25 - {x^2}} \) are solutions of \(dy/dx = - x/y\) on the interval \(( - 5,5)\). Explain why the piecewise-defined function \(y = \left\{ {\begin{aligned}{*{20}{c}}{\sqrt {25 - {x^2}} }&{ - 5 < x < 0}\\{ - \sqrt {25 - {x^2}} ,}&{0 \le x < 5}\end{aligned}} \right.\) is not a solution of the differential equation on the interval \(( - 5,5)\).
In Problems \(5\) and \(6\) compute \(y'\) and \(y''\) and then combine these derivatives with \(y\) as a linear second-order differential equation that is free of the symbols \({c_1}\) and \({c_2}\) and has the form \(F(y',y'',y''') = 0\). The symbols \({c_1}\) and \({c_2}\) represent constants.
\(y = {c_1}{e^x} + {c_2}x{e^x}\)
Raindrops Keep Falling In meteorology the term virga refers to falling raindrops or ice particles that evaporate before they reach the ground. Assume that a typical raindrop is spherical. Starting at some time, which we can designate as t = 0, the raindrop of radius r0 falls from rest from a cloud and begins to evaporate.
(a) If it is assumed that a raindrop evaporates in such a manner that its shape remains spherical, then it also makes sense to assume that the rate at which the raindrop evaporates—that is, the rate at which it loses mass—is proportional to its surface area. Show that this latter assumption implies that the rate at which the radius r of the raindrop decreases is a constant. Find r(t). (Hint: See Problem 55 in Exercises 1.1.)
(b) If the positive direction is downward, construct a mathematical model for the velocity v of the falling raindrop at time t > 0. Ignore air resistance. (Hint: Use the form of Newton’s second law of motion given in (17).)
In Problems \(35\) and \(36\) find values of m so that the function \(y = {m^x}\) is a solution of the given differential equation.
\(xy'' + 2y' = 0\)
In Problems \(1 - 8\( state the order of the given ordinary differential equation. Determine whether the equation is linear or nonlinear by matching it with \((6)\(.
\({\left( {\frac{{{d^2}y}}{{d{x^2}}}} \right)^2} = \sqrt {1 + {{\left( {\frac{{dy}}{{dx}}} \right)}^2}} \(
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