/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 24 From \(m^{3}-m^{2}-4 m+4=0\) we ... [FREE SOLUTION] | 91Ó°ÊÓ

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From \(m^{3}-m^{2}-4 m+4=0\) we find \(m_{1}=1, m_{2}=2,\) and \(m_{3}=-2 .\) Then \(y_{c}=c_{1} e^{x}+c_{2} e^{2 x}+c_{3} e^{-2 x}\) and we assume \(y_{p}=A+B x e^{x}+C x e^{2 x} .\) Substituting into the differential equation we obtain \(4 A=5,-3 B=-1,\) and \(4 C=1 .\) Then \(A=\frac{5}{4}, B=\frac{1}{3}, C=\frac{1}{4}, y_{p}=\frac{5}{4}+\frac{1}{3} x e^{x}+\frac{1}{4} x e^{2 x},\) and \(y=c_{1} e^{x}+c_{2} e^{2 x}+c_{3} e^{-2 x}+\frac{5}{4}+\frac{1}{3} x e^{x}+\frac{1}{4} x e^{2 x}\)

Short Answer

Expert verified
The general solution is \( y = c_1 e^{x} + c_2 e^{2x} + c_3 e^{-2x} + \frac{5}{4} + \frac{1}{3} x e^{x} + \frac{1}{4} x e^{2x} \).

Step by step solution

01

Identify Roots of the Given Equation

The given cubic equation is \( m^{3} - m^{2} - 4m + 4 = 0 \). From the problem, we know the roots are \( m_1 = 1 \), \( m_2 = 2 \), and \( m_3 = -2 \).
02

Write the Complementary Solution

Using the roots found, the complementary solution \( y_c \) of the differential equation based on these roots is \( y_c = c_1 e^{x} + c_2 e^{2x} + c_3 e^{-2x} \).
03

Assume a Particular Solution

The problem suggests the form of a particular solution as \( y_p = A + Bx e^{x} + Cx e^{2x} \). This is due to the presence of \( e^{x} \) and \( e^{2x} \) in the complementary solution, requiring terms \( x e^{x} \) and \( x e^{2x} \) for the particular solution.
04

Substitute into the Differential Equation

Substitute the assumed particular solution into the differential equation and collect like terms. The equating of coefficients gives rise to the system of equations: \( 4A = 5 \), \( -3B = -1 \), and \( 4C = 1 \).
05

Solve for Coefficients A, B, and C

Solving the system of equations from Step 4, we find \( A = \frac{5}{4} \), \( B = \frac{1}{3} \), and \( C = \frac{1}{4} \).
06

Write the Particular Solution

Using the values of \( A \), \( B \), and \( C \), the particular solution is \( y_p = \frac{5}{4} + \frac{1}{3} x e^{x} + \frac{1}{4} x e^{2x} \).
07

Combine to Find General Solution

The general solution of the differential equation is given by the sum of the complementary solution and the particular solution: \( y = c_1 e^{x} + c_2 e^{2x} + c_3 e^{-2x} + \frac{5}{4} + \frac{1}{3} x e^{x} + \frac{1}{4} x e^{2x} \).

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Key Concepts

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

Cubic Equation
Cubic equations are polynomial equations of degree three. They typically take the form: \[ ax^3 + bx^2 + cx + d = 0 \] where \(a\), \(b\), \(c\), and \(d\) are constants and \(a eq 0\). Solving a cubic equation involves finding the values of \(m\) (also known as roots or solutions) that satisfy the equation. In the given exercise, the cubic equation is \(m^3 - m^2 - 4m + 4 = 0\). To solve it, we might use methods such as factoring, synthetic division, or the Cubic Formula. In our case, the roots are already provided: \(m_1 = 1\), \(m_2 = 2\), and \(m_3 = -2\). This means these are the values of \(m\) for which the equation holds true, i.e., it equals zero when these roots are substituted back into the original equation.
Roots of Equation
The roots of an equation are the values that make the equation true when they replace the variable. For a cubic equation like \(m^3 - m^2 - 4m + 4 = 0\), the roots can often be found by strategic methods including factoring, the Rational Root Theorem, or direct computation. - In the given problem, the roots are given as \(m_1 = 1\), \(m_2 = 2\), and \(m_3 = -2\). - Each of these values represents a solution to the cubic equation where the left side equals zero. These roots are significant because they determine the form of the complementary solution to the related differential equation. Knowing the roots is crucial since each root corresponds to an exponential solution in the form of the differential equation's complementary solution.
Particular Solution
The particular solution of a non-homogeneous differential equation addresses the specific part of the solution that corresponds to the non-zero right-hand side of the equation. For a differential equation of the form \[ m^3 - m^2 - 4m + 4 = 0 \], we hypothesize a particular solution based on the non-homogeneous term. In our example, the particular solution is chosen as \[ y_p = A + Bx e^{x} + Cx e^{2x} \]. This guess is informed by the structure of the complementary solution and the form of the non-homogeneous function. By substituting this form into the differential equation, one can determine the specific values of \(A\), \(B\), and \(C\) by matching coefficients from both sides. This process yields the particular solution needed to complete the general solution of the differential equation.
Complementary Solution
The complementary solution, denoted as \(y_c\), of a homogeneous differential equation is derived from the associated homogeneous equation where all terms are equated to zero. In our exercise, the complementary solution takes the form: \[ y_c = c_1 e^{x} + c_2 e^{2x} + c_3 e^{-2x} \]. This result is based on the roots of the characteristic polynomial obtained from the differential equation, which in this case are \(m_1 = 1\), \(m_2 = 2\), and \(m_3 = -2\). Each root corresponds to an exponential term in the complementary solution. - For \(m_1 = 1\), the term is \(c_1 e^{x}\). - For \(m_2 = 2\), it is \(c_2 e^{2x}\). - For \(m_3 = -2\), it is \(c_3 e^{-2x}\). By then adjusting for specific initial or boundary conditions, one can solve for the coefficients \(c_1\), \(c_2\), and \(c_3\), completing the complementary part of the general solution to the differential equation.

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Most popular questions from this chapter

Solving \(\frac{1}{2} q^{\prime \prime}+20 q^{\prime}+1000 q=0\) we obtain \(q_{c}(t)=e^{-20 t}\left(c_{1} \cos 40 t+c_{2} \sin 40 t\right) .\) The steady- state charge has the form \(q_{p}(t)=A \sin 60 t+B \cos 60 t+C \sin 40 t+D \cos 40 t .\) Substituting into the differential equation we find $$\begin{aligned}(-1600 A-2400 B) & \sin 60 t+(2400 A-1600 B) \cos 60 t \\ +&(400 C-1600 D) \sin 40 t+(1600 C+400 D) \cos 40 t\end{aligned}$$ $$=200 \sin 60 t+400 \cos 40 t$$ Equating coefficients we obtain \(A=-1 / 26, B=-3 / 52, C=4 / 17,\) and \(D=1 / 17 .\) The steady-state charge is \\[q_{p}(t)=-\frac{1}{26} \sin 60 t-\frac{3}{52} \cos 60 t+\frac{4}{17} \sin 40 t+\frac{1}{17} \cos 40 t\\] and the steady-state current is \\[i_{p}(t)=-\frac{30}{13} \cos 60 t+\frac{45}{13} \sin 60 t+\frac{160}{17} \cos 40 t-\frac{40}{17} \sin 40 t\\].

From \(\left(D^{2}+5\right) x-2 y=0\) and \(-2 x+\left(D^{2}+2\right) y=0\) we obtain \(y=\frac{1}{2}\left(D^{2}+5\right) x, D^{2} y=\frac{1}{2}\left(D^{4}+5 D^{2}\right) x,\) and \(\left(D^{2}+1\right)\left(D^{2}+6\right) x=0 .\) The solution is $$\begin{aligned} &x=c_{1} \cos t+c_{2} \sin t+c_{3} \cos \sqrt{6} t+c_{4} \sin \sqrt{6} t\\\ &y=2 c_{1} \cos t+2 c_{2} \sin t-\frac{1}{2} c_{3} \cos \sqrt{6} t-\frac{1}{2} c_{4} \sin \sqrt{6} t \end{aligned}.$$

If \(1-i\) is a root of the auxiliary equation then so is \(1+i,\) and the auxiliary equation is $$(m-2)[m-(1+i)][m-(1-i)]=m^{3}-4 m^{2}+6 m-4=0.$$ We need \(m^{3}-4 m^{2}+6 m-4\) to have the form \(m(m-1)(m-2)+b m(m-1)+c m+d .\) Expanding this last expression and equating coefficients we get \(b=-1, c=3,\) and \(d=-4 .\) Thus, the differential equation is $$x^{3} y^{\prime \prime \prime}-x^{2} y^{\prime \prime}+3 x y^{\prime}-4 y=0.$$

$$\text { since } x=\frac{\sqrt{85}}{4} \sin (4 t-0.219)-\frac{\sqrt{17}}{2} e^{-2 t} \sin (4 t-2.897), \text { the amplitude approaches } \sqrt{85} / 4 \text { as } t \rightarrow \infty$$.

For large values of \(t\) the differential equation is approximated by \(x^{\prime \prime}=0 .\) The solution of this equation is the linear function \(x=c_{1} t+c_{2} .\) Thus, for large time, the restoring force will have decayed to the point where the spring is incapable of returning the mass, and the spring will simply keep on stretching.

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