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In Problems 1-18, solve the given differential equation. $$ 4 x^{2} y^{\prime \prime}+4 x y^{\prime}-y=0 $$

Short Answer

Expert verified
The general solution is \( y(x) = C_1 x^{\frac{-1 + \sqrt{2}}{2}} + C_2 x^{\frac{-1 - \sqrt{2}}{2}} \).

Step by step solution

01

Identify the Type of Equation

The given differential equation is a second-order linear homogeneous differential equation with variable coefficients: \( 4x^2 y'' + 4xy' - y = 0 \).
02

Standard Form and Euler-Cauchy Equation

Recognize the equation as an Euler-Cauchy equation, which has the form \( x^2y'' + axy' + by = 0 \). By dividing the entire equation by 4, we obtain the standard form: \( x^2 y'' + x y' - \frac{1}{4} y = 0 \).
03

Solve Characteristic Equation

Assume a solution of the form \( y = x^m \). Calculate the derivatives: \( y' = mx^{m-1} \) and \( y'' = m(m-1)x^{m-2} \). Substitute these into the standard form equation and gather like terms: \( x^2(m(m-1)x^{m-2}) + x(mx^{m-1}) - \frac{1}{4}x^m = 0 \). Simplifying this gives the characteristic equation: \( m(m-1) + m - \frac{1}{4} = 0 \).
04

Solve Characteristic Equation for m

Solve the characteristic equation \( m^2 \pm m - \frac{1}{4} = 0 \) using the quadratic formula, \( m = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \), where \( a = 1, b = 1, c = -\frac{1}{4} \). Calculating the discriminant \( b^2 - 4ac = 1 + 1 = 2 \), we find \( m = \frac{-1 \pm \sqrt{2}}{2} \).
05

Write General Solution

For the roots \( m_1 = \frac{-1 + \sqrt{2}}{2} \) and \( m_2 = \frac{-1 - \sqrt{2}}{2} \), the general solution of the differential equation is: \( y(x) = C_1 x^{m_1} + C_2 x^{m_2} \). This can be rewritten as: \( y(x) = C_1 x^{\frac{-1 + \sqrt{2}}{2}} + C_2 x^{\frac{-1 - \sqrt{2}}{2}} \), where \( C_1 \) and \( C_2 \) are constants.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

second-order linear differential equation
A second-order linear differential equation is a powerful tool in mathematics, primarily used to describe various dynamic systems. These equations involve a mathematical expression containing the second derivative of a function. In the case of the given differential equation, \(4x^2 y'' + 4xy' - y = 0\), the highest derivative present is \(y''\), which makes this equation a second-order equation. Just like how first-order differential equations model rates of change (think speed from distance over time), second-order ones model acceleration, as they include terms like \(y''\). This is notably useful in physics for modeling things like oscillations and vibrations. Linear differential equations also imply that the function and its derivatives appear to the first power, allowing for a wide array of techniques for finding solutions. A unique characteristic of these equations is their homogeneity, meaning they contain no terms independent of the dependent variable and its derivatives, ensuring simpler solutions.
characteristic equation
The characteristic equation is a key concept when solving linear differential equations, particularly Euler-Cauchy equations. To simplify finding solutions to these differential equations, we use the characteristic equation, which comes from assuming solutions of a specific form.For the differential equation \(x^2 y'' + ax y' + by = 0\), we can try a solution of the form \(y = x^m\). Then, derive \(y' = mx^{m-1}\) and \(y'' = m(m-1)x^{m-2}\). Plug these back into the differential equation, leading to an algebraic form known as the characteristic equation.In our problem, it resulted in the quadratic form \(m^2 + m - \frac{1}{4} = 0\). Solving this characteristically involves the quadratic formula \(m = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\). Solving such a characteristic equation gives us the possible values \(m\) can take, which are crucial for determining the general solution of the differential equation.
general solution of differential equations
The general solution of a differential equation represents the broadest set of possible solutions. This includes possible constants and functions that satisfy the original differential equation. In our scenario, the solution arises from solving the characteristic equation.The roots \(m_1\) and \(m_2\) found from the characteristic equation directly influence the form of the general solution. For the second-order Euler-Cauchy differential equation, these roots generally lead you to a solution of the form \(y(x) = C_1 x^{m_1} + C_2 x^{m_2}\). Here, \(C_1\) and \(C_2\) are arbitrary constants that can be adjusted depending on specific initial or boundary conditions. This flexibility is what defines the 'generality' of the solution.If the roots are real and distinct, the form remains straightforward, as it is in our exercise, where we have found \(y(x) = C_1 x^{\frac{-1 + \sqrt{2}}{2}} + C_2 x^{\frac{-1 - \sqrt{2}}{2}}\). This allows for a wide range of scenarios to be modeled by merely adjusting \(C_1\) and \(C_2\) accordingly.

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Most popular questions from this chapter

Find a homogeneous linear differential equation with constant coefficients whose general solution is given. $$ y \quad c_{1} e^{-5 x}+c_{2} e^{-4 x} $$

Solve the given differential equation by undetermined coefficients. \(y^{\prime \prime}+2 y^{\prime}-24 y=16-(x+2) e^{4 x}\)

Discuss how to find an alternative two-parameter family of solutions for the nonlinear differential equation \(y^{\prime \prime}=2 x\left(y^{\prime}\right)^{2}\) in Example 1. [Hint: Suppose that \(-c_{1}^{2}\) is used as the constant of integration instead of \(\left.+c_{1}^{2} .\right]\)

Consider a pendulum that is released from rest from an initial displacement of \(\theta_{0}\) radians. Solving the linear model (7) subject to the initial conditions \(\theta(0)=\theta_{0}, \theta^{\prime}(0)=0\) gives \(\theta(t)=\theta_{0} \cos \sqrt{g} \| t .\) Theperiod of oscillations predicted by this modelisgivenbythefamiliarformula \(T=2 \pi / \sqrt{g l l}=2 \pi \sqrt{U g}\). The interesting thing about this formula for \(T\) is that it does not depend on the magnitude of the initial displacement \(\theta_{0}\). In other words, the linear model predicts that the time that it would take the pendulum to swing from an initial displacement of, say, \(\theta_{0}=\pi / 2\left(=90^{\circ}\right)\) to \(-\pi / 2\) and back again would be exactly the same time to cycle from, say, \(\theta_{0}=\pi / 360\left(=0.5^{\circ}\right)\) to \(-\pi / 360\). This is intuitively unreasonable; the actual period must depend on \(\theta_{0}\). If we assume that \(g=32 \mathrm{ft} / \mathrm{s}^{2}\) and \(l=32 \mathrm{ft}\), then the period of oscillation of the linear model is \(T=2 \pi \mathrm{s}\). Let us compare this last number with the period predicted by the nonlinear model when \(\theta_{0}=\pi / 4\). Using a numerical solver that is capable of generating hard data, approximate the solution of $$\frac{d^{2} \theta}{d t^{2}}+\sin \theta=0, \quad \theta(0)=\frac{\pi}{4}, \quad \theta^{\prime}(0)=0$$ for \(0 \leq t \leq 2\). As in Problem 24 , if \(t_{1}\) denotes the first time the pendulum reaches the position \(O P\) in Figure 3.11.3, then the period of the nonlinear pendulum is \(4 t_{1} .\) Here is another way of solving the equation \(\theta(t)=0\). Expeniment with small step sizes and advance the time staning at \(t=0\) and ending at \(t=2\). From your hard data, observe the time \(t_{1}\) when \(\theta(t)\) changes, for the first time, from positive to negative. Use the value \(t_{1}\) to determine the true value of the period of the nonlinear pendulum. Compute the percentage relative error in the period estimated by \(T=2 \pi\).

Solve the given initial-value problem. \(5 y^{\prime \prime}+y^{\prime}=-6 x, y(0)=0, y^{\prime}(0)=-10\)

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