Chapter 8: Problem 3
Consider the initial value problem \(y^{\prime}(t)=t^{2}-3 y^{2}, y(3)=1\) What is the approximation to \(y(3.1)\) given by Euler's method with a time step of \(\Delta t=0.1 ?\)
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Chapter 8: Problem 3
Consider the initial value problem \(y^{\prime}(t)=t^{2}-3 y^{2}, y(3)=1\) What is the approximation to \(y(3.1)\) given by Euler's method with a time step of \(\Delta t=0.1 ?\)
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Determine whether the following statements are true and give an explanation or counterexample. a. The general solution of \(y^{\prime}(t)=2 y-18\) is \(y(t)=2 e^{2 t}+9\) b. If \(k>0\) and \(b>0,\) then \(y(t)=0\) is never a solution of \(y^{\prime}(t)=k y-b\) c. The equation \(y^{\prime}(t)=t y(t)+3\) is separable and can be solved using the methods of this section. d. According to Newton's Law of Cooling, the temperature of a hot object will reach the ambient temperature after a finite amount of time.
RC circuit equation Suppose a battery with voltage \(V\) is connected in series to a capacitor (a charge storage device) with capacitance \(C\) and a resistor with resistance \(R\). As the charge \(Q\) in the capacitor increases, the current \(I\) across the capacitor decreases according to the following initial value problems. Solve each initial value problem and interpret the solution. a. \(I^{\prime}(t)+\frac{1}{R C} I(t)=0, I(0)=\frac{V}{R}\) b. \(Q^{\prime}(t)+\frac{1}{R C} Q(t)=\frac{V}{R}, Q(0)=0\)
Make a sketch of the population function (as a function of time) that results from the following growth rate functions. Assume the population at time \(t=0\) begins at some positive value.
Consider the general first-order linear equation \(y^{\prime}(t)+a(t) y(t)=f(t) .\) This equation can be solved, in principle, by defining the integrating factor \(p(t)=\exp \left(\int a(t) d t\right) .\) Here is how the integrating factor works. Multiply both sides of the equation by \(p\) (which is always positive) and show that the left side becomes an exact derivative. Therefore, the equation becomes $$p(t)\left(y^{\prime}(t)+a(t) y(t)\right)=\frac{d}{d t}(p(t) y(t))=p(t) f(t).$$ Now integrate both sides of the equation with respect to t to obtain the solution. Use this method to solve the following initial value problems. Begin by computing the required integrating factor. $$y^{\prime}(t)+\frac{2 t}{t^{2}+1} y(t)=1+3 t^{2}, \quad y(1)=4$$
Solve the differential equation for Newton's Law of Cooling to find the temperature in the following cases. Then answer any additional questions. A cup of coffee has a temperature of \(90^{\circ} \mathrm{C}\) when it is poured and allowed to cool in a room with a temperature of \(25^{\circ} \mathrm{C}\). One minute after the coffee is poured, its temperature is \(85^{\circ} \mathrm{C}\). How long must you wait until the coffee is cool enough to drink, say \(30^{\circ} \mathrm{C} ?\)
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