problem 10

Internal problem ID [12801]
Internal ﬁle name [OUTPUT/11454_Saturday_November_04_2023_08_47_24_AM_46660882/index.tex]

Book: Ordinary Diﬀerential Equations by Charles E. Roberts, Jr. CRC Press. 2010
Section: Chapter 5. The Laplace Transform Method. Exercises 5.3, page 255
Problem number: 10.
ODE order: 2.
ODE degree: 1.

The type(s) of ODE detected by this program : "second_order_laplace", "second_order_linear_constant_coeﬀ"

\[ \boxed {y^{\prime \prime }+9 y=18 \,{\mathrm e}^{3 x}} \] With initial conditions \begin {align*} [y \left (0\right ) = -1, y^{\prime }\left (0\right ) = 6] \end {align*}

14.4.1 Existence and uniqueness analysis

This is a linear ODE. In canonical form it is written as \begin {align*} y^{\prime \prime } + p(x)y^{\prime } + q(x) y &= F \end {align*}

Where here \begin {align*} p(x) &=0\\ q(x) &=9\\ F &=18 \,{\mathrm e}^{3 x} \end {align*}

Hence the ode is \begin {align*} y^{\prime \prime }+9 y = 18 \,{\mathrm e}^{3 x} \end {align*}

Solving using the Laplace transform method. Let \begin {align*} \mathcal {L}\left (y\right ) =Y(s) \end {align*}

Taking the Laplace transform of the ode and using the relations that \begin {align*} \mathcal {L}\left (y^{\prime }\right ) &= s Y(s) - y \left (0\right )\\ \mathcal {L}\left (y^{\prime \prime }\right ) &= s^2 Y(s) - y'(0) - s y \left (0\right ) \end {align*}

The given ode now becomes an algebraic equation in the Laplace domain \begin {align*} s^{2} Y \left (s \right )-y^{\prime }\left (0\right )-s y \left (0\right )+9 Y \left (s \right ) = \frac {18}{s -3}\tag {1} \end {align*}

But the initial conditions are \begin {align*} y \left (0\right )&=-1\\ y'(0) &=6 \end {align*}

Substituting these initial conditions in above in Eq (1) gives \begin {align*} s^{2} Y \left (s \right )-6+s +9 Y \left (s \right ) = \frac {18}{s -3} \end {align*}

Solving the above equation for \(Y(s)\) results in \begin {align*} Y(s) = -\frac {s \left (s -9\right )}{\left (s -3\right ) \left (s^{2}+9\right )} \end {align*}

Applying partial fractions decomposition results in \[ Y(s)= \frac {1}{s -3}+\frac {-1-\frac {i}{2}}{s -3 i}+\frac {-1+\frac {i}{2}}{s +3 i} \] The inverse Laplace of each term above is now found, which gives \begin {align*} \mathcal {L}^{-1}\left (\frac {1}{s -3}\right ) &= {\mathrm e}^{3 x}\\ \mathcal {L}^{-1}\left (\frac {-1-\frac {i}{2}}{s -3 i}\right ) &= \left (-1-\frac {i}{2}\right ) {\mathrm e}^{3 i x}\\ \mathcal {L}^{-1}\left (\frac {-1+\frac {i}{2}}{s +3 i}\right ) &= \left (-1+\frac {i}{2}\right ) {\mathrm e}^{-3 i x} \end {align*}

Adding the above results and simplifying gives \[ y=-2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \] Simplifying the solution gives \[ y = -2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \]

Summary

The solution(s) found are the following \begin{align*} \tag{1} y &= -2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \\ \end{align*}

Veriﬁcation of solutions

\[ y = -2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \] Veriﬁed OK.

14.4.2 Maple step by step solution

\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & \left [y^{\prime \prime }+9 y=18 \,{\mathrm e}^{3 x}, y \left (0\right )=-1, y^{\prime }{\raise{-0.36em}{\Big |}}{\mstack {}{_{\left \{x \hiderel {=}0\right \}}}}=6\right ] \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 2 \\ {} & {} & y^{\prime \prime } \\ \bullet & {} & \textrm {Characteristic polynomial of homogeneous ODE}\hspace {3pt} \\ {} & {} & r^{2}+9=0 \\ \bullet & {} & \textrm {Use quadratic formula to solve for}\hspace {3pt} r \\ {} & {} & r =\frac {0\pm \left (\sqrt {-36}\right )}{2} \\ \bullet & {} & \textrm {Roots of the characteristic polynomial}\hspace {3pt} \\ {} & {} & r =\left (-3 \,\mathrm {I}, 3 \,\mathrm {I}\right ) \\ \bullet & {} & \textrm {1st solution of the homogeneous ODE}\hspace {3pt} \\ {} & {} & y_{1}\left (x \right )=\cos \left (3 x \right ) \\ \bullet & {} & \textrm {2nd solution of the homogeneous ODE}\hspace {3pt} \\ {} & {} & y_{2}\left (x \right )=\sin \left (3 x \right ) \\ \bullet & {} & \textrm {General solution of the ODE}\hspace {3pt} \\ {} & {} & y=c_{1} y_{1}\left (x \right )+c_{2} y_{2}\left (x \right )+y_{p}\left (x \right ) \\ \bullet & {} & \textrm {Substitute in solutions of the homogeneous ODE}\hspace {3pt} \\ {} & {} & y=\cos \left (3 x \right ) c_{1} +c_{2} \sin \left (3 x \right )+y_{p}\left (x \right ) \\ \square & {} & \textrm {Find a particular solution}\hspace {3pt} y_{p}\left (x \right )\hspace {3pt}\textrm {of the ODE}\hspace {3pt} \\ {} & \circ & \textrm {Use variation of parameters to find}\hspace {3pt} y_{p}\hspace {3pt}\textrm {here}\hspace {3pt} f \left (x \right )\hspace {3pt}\textrm {is the forcing function}\hspace {3pt} \\ {} & {} & \left [y_{p}\left (x \right )=-y_{1}\left (x \right ) \left (\int \frac {y_{2}\left (x \right ) f \left (x \right )}{W \left (y_{1}\left (x \right ), y_{2}\left (x \right )\right )}d x \right )+y_{2}\left (x \right ) \left (\int \frac {y_{1}\left (x \right ) f \left (x \right )}{W \left (y_{1}\left (x \right ), y_{2}\left (x \right )\right )}d x \right ), f \left (x \right )=18 \,{\mathrm e}^{3 x}\right ] \\ {} & \circ & \textrm {Wronskian of solutions of the homogeneous equation}\hspace {3pt} \\ {} & {} & W \left (y_{1}\left (x \right ), y_{2}\left (x \right )\right )=\left [\begin {array}{cc} \cos \left (3 x \right ) & \sin \left (3 x \right ) \\ -3 \sin \left (3 x \right ) & 3 \cos \left (3 x \right ) \end {array}\right ] \\ {} & \circ & \textrm {Compute Wronskian}\hspace {3pt} \\ {} & {} & W \left (y_{1}\left (x \right ), y_{2}\left (x \right )\right )=3 \\ {} & \circ & \textrm {Substitute functions into equation for}\hspace {3pt} y_{p}\left (x \right ) \\ {} & {} & y_{p}\left (x \right )=-6 \cos \left (3 x \right ) \left (\int \sin \left (3 x \right ) {\mathrm e}^{3 x}d x \right )+6 \sin \left (3 x \right ) \left (\int \cos \left (3 x \right ) {\mathrm e}^{3 x}d x \right ) \\ {} & \circ & \textrm {Compute integrals}\hspace {3pt} \\ {} & {} & y_{p}\left (x \right )={\mathrm e}^{3 x} \\ \bullet & {} & \textrm {Substitute particular solution into general solution to ODE}\hspace {3pt} \\ {} & {} & y=\cos \left (3 x \right ) c_{1} +c_{2} \sin \left (3 x \right )+{\mathrm e}^{3 x} \\ \square & {} & \textrm {Check validity of solution}\hspace {3pt} y=\cos \left (3 x \right ) c_{1} +c_{2} \sin \left (3 x \right )+{\mathrm e}^{3 x} \\ {} & \circ & \textrm {Use initial condition}\hspace {3pt} y \left (0\right )=-1 \\ {} & {} & -1=1+c_{1} \\ {} & \circ & \textrm {Compute derivative of the solution}\hspace {3pt} \\ {} & {} & y^{\prime }=-3 \sin \left (3 x \right ) c_{1} +3 c_{2} \cos \left (3 x \right )+3 \,{\mathrm e}^{3 x} \\ {} & \circ & \textrm {Use the initial condition}\hspace {3pt} y^{\prime }{\raise{-0.36em}{\Big |}}{\mstack {}{_{\left \{x \hiderel {=}0\right \}}}}=6 \\ {} & {} & 6=3+3 c_{2} \\ {} & \circ & \textrm {Solve for}\hspace {3pt} c_{1} \hspace {3pt}\textrm {and}\hspace {3pt} c_{2} \\ {} & {} & \left \{c_{1} =-2, c_{2} =1\right \} \\ {} & \circ & \textrm {Substitute constant values into general solution and simplify}\hspace {3pt} \\ {} & {} & y=-2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \\ \bullet & {} & \textrm {Solution to the IVP}\hspace {3pt} \\ {} & {} & y=-2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \end {array} \]

Maple trace

`Methods for second order ODEs: 
--- Trying classification methods --- 
trying a quadrature 
trying high order exact linear fully integrable 
trying differential order: 2; linear nonhomogeneous with symmetry [0,1] 
trying a double symmetry of the form [xi=0, eta=F(x)] 
-> Try solving first the homogeneous part of the ODE 
   checking if the LODE has constant coefficients 
   <- constant coefficients successful 
<- solving first the homogeneous part of the ODE successful`

\[ y \left (x \right ) = -2 \cos \left (3 x \right )+\sin \left (3 x \right )+{\mathrm e}^{3 x} \]

14.4 problem 10

14.4.1 Existence and uniqueness analysis

14.4.2 Maple step by step solution