11.5 problem 30

Internal problem ID [5396]
Internal file name [OUTPUT/4887_Sunday_February_04_2024_12_46_49_AM_92657304/index.tex]

Book: Schaums Outline. Theory and problems of Differential Equations, 1st edition. Frank Ayres. McGraw Hill 1952
Section: Chapter 16. Linear equations with constant coefficients (Short methods). Supplemetary problems. Page 107
Problem number: 30.
ODE order: 3.
ODE degree: 1.

The type(s) of ODE detected by this program : "higher_order_linear_constant_coefficients_ODE"

Maple gives the following as the ode type

[[_3rd_order, _linear, _nonhomogeneous]]

\[ \boxed {y^{\prime \prime \prime }+y=\cos \left (x \right )} \] This is higher order nonhomogeneous ODE. Let the solution be \[ y = y_h + y_p \] Where \(y_h\) is the solution to the homogeneous ODE And \(y_p\) is a particular solution to the nonhomogeneous ODE. \(y_h\) is the solution to \[ y^{\prime \prime \prime }+y = 0 \] The characteristic equation is \[ \lambda ^{3}+1 = 0 \] The roots of the above equation are \begin {align*} \lambda _1 &= -1\\ \lambda _2 &= \frac {1}{2}-\frac {i \sqrt {3}}{2}\\ \lambda _3 &= \frac {1}{2}+\frac {i \sqrt {3}}{2} \end {align*}

Therefore the homogeneous solution is \[ y_h(x)=c_{1} {\mathrm e}^{-x}+{\mathrm e}^{\left (\frac {1}{2}+\frac {i \sqrt {3}}{2}\right ) x} c_{2} +{\mathrm e}^{\left (\frac {1}{2}-\frac {i \sqrt {3}}{2}\right ) x} c_{3} \] The fundamental set of solutions for the homogeneous solution are the following \begin{align*} y_1 &= {\mathrm e}^{-x} \\ y_2 &= {\mathrm e}^{\left (\frac {1}{2}+\frac {i \sqrt {3}}{2}\right ) x} \\ y_3 &= {\mathrm e}^{\left (\frac {1}{2}-\frac {i \sqrt {3}}{2}\right ) x} \\ \end{align*} Now the particular solution to the given ODE is found \[ y^{\prime \prime \prime }+y = \cos \left (x \right ) \] The particular solution is found using the method of undetermined coefficients. Looking at the RHS of the ode, which is \[ \cos \left (x \right ) \] Shows that the corresponding undetermined set of the basis functions (UC_set) for the trial solution is \[ [\{\cos \left (x \right ), \sin \left (x \right )\}] \] While the set of the basis functions for the homogeneous solution found earlier is \[ \left \{{\mathrm e}^{\left (\frac {1}{2}-\frac {i \sqrt {3}}{2}\right ) x}, {\mathrm e}^{\left (\frac {1}{2}+\frac {i \sqrt {3}}{2}\right ) x}, {\mathrm e}^{-x}\right \} \] Since there is no duplication between the basis function in the UC_set and the basis functions of the homogeneous solution, the trial solution is a linear combination of all the basis in the UC_set. \[ y_p = A_{1} \cos \left (x \right )+A_{2} \sin \left (x \right ) \] The unknowns \(\{A_{1}, A_{2}\}\) are found by substituting the above trial solution \(y_p\) into the ODE and comparing coefficients. Substituting the trial solution into the ODE and simplifying gives \[ A_{1} \sin \left (x \right )-A_{2} \cos \left (x \right )+A_{1} \cos \left (x \right )+A_{2} \sin \left (x \right ) = \cos \left (x \right ) \] Solving for the unknowns by comparing coefficients results in \[ \left [A_{1} = {\frac {1}{2}}, A_{2} = -{\frac {1}{2}}\right ] \] Substituting the above back in the above trial solution \(y_p\), gives the particular solution \[ y_p = \frac {\cos \left (x \right )}{2}-\frac {\sin \left (x \right )}{2} \] Therefore the general solution is \begin{align*} y &= y_h + y_p \\ &= \left (c_{1} {\mathrm e}^{-x}+{\mathrm e}^{\left (\frac {1}{2}+\frac {i \sqrt {3}}{2}\right ) x} c_{2} +{\mathrm e}^{\left (\frac {1}{2}-\frac {i \sqrt {3}}{2}\right ) x} c_{3}\right ) + \left (\frac {\cos \left (x \right )}{2}-\frac {\sin \left (x \right )}{2}\right ) \\ \end{align*}

Summary

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

Verification of solutions

\[ y = c_{1} {\mathrm e}^{-x}+{\mathrm e}^{\left (\frac {1}{2}+\frac {i \sqrt {3}}{2}\right ) x} c_{2} +{\mathrm e}^{\left (\frac {1}{2}-\frac {i \sqrt {3}}{2}\right ) x} c_{3} +\frac {\cos \left (x \right )}{2}-\frac {\sin \left (x \right )}{2} \] Verified OK.

11.5.1 Maple step by step solution

\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & \frac {d}{d x}y^{\prime \prime }+y=\cos \left (x \right ) \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 3 \\ {} & {} & \frac {d}{d x}y^{\prime \prime } \\ \square & {} & \textrm {Convert linear ODE into a system of first order ODEs}\hspace {3pt} \\ {} & \circ & \textrm {Define new variable}\hspace {3pt} y_{1}\left (x \right ) \\ {} & {} & y_{1}\left (x \right )=y \\ {} & \circ & \textrm {Define new variable}\hspace {3pt} y_{2}\left (x \right ) \\ {} & {} & y_{2}\left (x \right )=y^{\prime } \\ {} & \circ & \textrm {Define new variable}\hspace {3pt} y_{3}\left (x \right ) \\ {} & {} & y_{3}\left (x \right )=\frac {d}{d x}y^{\prime } \\ {} & \circ & \textrm {Isolate for}\hspace {3pt} y_{3}^{\prime }\left (x \right )\hspace {3pt}\textrm {using original ODE}\hspace {3pt} \\ {} & {} & y_{3}^{\prime }\left (x \right )=\cos \left (x \right )-y_{1}\left (x \right ) \\ & {} & \textrm {Convert linear ODE into a system of first order ODEs}\hspace {3pt} \\ {} & {} & \left [y_{2}\left (x \right )=y_{1}^{\prime }\left (x \right ), y_{3}\left (x \right )=y_{2}^{\prime }\left (x \right ), y_{3}^{\prime }\left (x \right )=\cos \left (x \right )-y_{1}\left (x \right )\right ] \\ \bullet & {} & \textrm {Define vector}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}\left (x \right )=\left [\begin {array}{c} y_{1}\left (x \right ) \\ y_{2}\left (x \right ) \\ y_{3}\left (x \right ) \end {array}\right ] \\ \bullet & {} & \textrm {System to solve}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}^{\prime }\left (x \right )=\left [\begin {array}{ccc} 0 & 1 & 0 \\ 0 & 0 & 1 \\ -1 & 0 & 0 \end {array}\right ]\cdot {\moverset {\rightarrow }{y}}\left (x \right )+\left [\begin {array}{c} 0 \\ 0 \\ \cos \left (x \right ) \end {array}\right ] \\ \bullet & {} & \textrm {Define the forcing function}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{f}}\left (x \right )=\left [\begin {array}{c} 0 \\ 0 \\ \cos \left (x \right ) \end {array}\right ] \\ \bullet & {} & \textrm {Define the coefficient matrix}\hspace {3pt} \\ {} & {} & A =\left [\begin {array}{ccc} 0 & 1 & 0 \\ 0 & 0 & 1 \\ -1 & 0 & 0 \end {array}\right ] \\ \bullet & {} & \textrm {Rewrite the system as}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}^{\prime }\left (x \right )=A \cdot {\moverset {\rightarrow }{y}}\left (x \right )+{\moverset {\rightarrow }{f}} \\ \bullet & {} & \textrm {To solve the system, find the eigenvalues and eigenvectors of}\hspace {3pt} A \\ \bullet & {} & \textrm {Eigenpairs of}\hspace {3pt} A \\ {} & {} & \left [\left [-1, \left [\begin {array}{c} 1 \\ -1 \\ 1 \end {array}\right ]\right ], \left [\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}, \left [\begin {array}{c} \frac {1}{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {1}{\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}} \\ 1 \end {array}\right ]\right ], \left [\frac {1}{2}+\frac {\mathrm {I} \sqrt {3}}{2}, \left [\begin {array}{c} \frac {1}{\left (\frac {1}{2}+\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {1}{\frac {1}{2}+\frac {\mathrm {I} \sqrt {3}}{2}} \\ 1 \end {array}\right ]\right ]\right ] \\ \bullet & {} & \textrm {Consider eigenpair}\hspace {3pt} \\ {} & {} & \left [-1, \left [\begin {array}{c} 1 \\ -1 \\ 1 \end {array}\right ]\right ] \\ \bullet & {} & \textrm {Solution to homogeneous system from eigenpair}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}_{1}={\mathrm e}^{-x}\cdot \left [\begin {array}{c} 1 \\ -1 \\ 1 \end {array}\right ] \\ \bullet & {} & \textrm {Consider complex eigenpair, complex conjugate eigenvalue can be ignored}\hspace {3pt} \\ {} & {} & \left [\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}, \left [\begin {array}{c} \frac {1}{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {1}{\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}} \\ 1 \end {array}\right ]\right ] \\ \bullet & {} & \textrm {Solution from eigenpair}\hspace {3pt} \\ {} & {} & {\mathrm e}^{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right ) x}\cdot \left [\begin {array}{c} \frac {1}{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {1}{\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}} \\ 1 \end {array}\right ] \\ \bullet & {} & \textrm {Use Euler identity to write solution in terms of}\hspace {3pt} \sin \hspace {3pt}\textrm {and}\hspace {3pt} \cos \\ {} & {} & {\mathrm e}^{\frac {x}{2}}\cdot \left (\cos \left (\frac {\sqrt {3}\, x}{2}\right )-\mathrm {I} \sin \left (\frac {\sqrt {3}\, x}{2}\right )\right )\cdot \left [\begin {array}{c} \frac {1}{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {1}{\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}} \\ 1 \end {array}\right ] \\ \bullet & {} & \textrm {Simplify expression}\hspace {3pt} \\ {} & {} & {\mathrm e}^{\frac {x}{2}}\cdot \left [\begin {array}{c} \frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )-\mathrm {I} \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{\left (\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}\right )^{2}} \\ \frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )-\mathrm {I} \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{\frac {1}{2}-\frac {\mathrm {I} \sqrt {3}}{2}} \\ \cos \left (\frac {\sqrt {3}\, x}{2}\right )-\mathrm {I} \sin \left (\frac {\sqrt {3}\, x}{2}\right ) \end {array}\right ] \\ \bullet & {} & \textrm {Both real and imaginary parts are solutions to the homogeneous system}\hspace {3pt} \\ {} & {} & \left [{\moverset {\rightarrow }{y}}_{2}\left (x \right )={\mathrm e}^{\frac {x}{2}}\cdot \left [\begin {array}{c} -\frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2} \\ \frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2} \\ \cos \left (\frac {\sqrt {3}\, x}{2}\right ) \end {array}\right ], {\moverset {\rightarrow }{y}}_{3}\left (x \right )={\mathrm e}^{\frac {x}{2}}\cdot \left [\begin {array}{c} \frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2} \\ \frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}-\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2} \\ -\sin \left (\frac {\sqrt {3}\, x}{2}\right ) \end {array}\right ]\right ] \\ \bullet & {} & \textrm {General solution of the system of ODEs can be written in terms of the particular solution}\hspace {3pt} {\moverset {\rightarrow }{y}}_{p}\left (x \right ) \\ {} & {} & {\moverset {\rightarrow }{y}}\left (x \right )=c_{1} {\moverset {\rightarrow }{y}}_{1}+c_{2} {\moverset {\rightarrow }{y}}_{2}\left (x \right )+c_{3} {\moverset {\rightarrow }{y}}_{3}\left (x \right )+{\moverset {\rightarrow }{y}}_{p}\left (x \right ) \\ \square & {} & \textrm {Fundamental matrix}\hspace {3pt} \\ {} & \circ & \textrm {Let}\hspace {3pt} \phi \left (x \right )\hspace {3pt}\textrm {be the matrix whose columns are the independent solutions of the homogeneous system.}\hspace {3pt} \\ {} & {} & \phi \left (x \right )=\left [\begin {array}{ccc} {\mathrm e}^{-x} & {\mathrm e}^{\frac {x}{2}} \left (-\frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) & {\mathrm e}^{\frac {x}{2}} \left (\frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) \\ -{\mathrm e}^{-x} & {\mathrm e}^{\frac {x}{2}} \left (\frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) & {\mathrm e}^{\frac {x}{2}} \left (\frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}-\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) \\ {\mathrm e}^{-x} & \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {x}{2}} & -\sin \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {x}{2}} \end {array}\right ] \\ {} & \circ & \textrm {The fundamental matrix,}\hspace {3pt} \Phi \left (x \right )\hspace {3pt}\textrm {is a normalized version of}\hspace {3pt} \phi \left (x \right )\hspace {3pt}\textrm {satisfying}\hspace {3pt} \Phi \left (0\right )=I \hspace {3pt}\textrm {where}\hspace {3pt} I \hspace {3pt}\textrm {is the identity matrix}\hspace {3pt} \\ {} & {} & \Phi \left (x \right )=\phi \left (x \right )\cdot \phi \left (0\right )^{-1} \\ {} & \circ & \textrm {Substitute the value of}\hspace {3pt} \phi \left (x \right )\hspace {3pt}\textrm {and}\hspace {3pt} \phi \left (0\right ) \\ {} & {} & \Phi \left (x \right )=\left [\begin {array}{ccc} {\mathrm e}^{-x} & {\mathrm e}^{\frac {x}{2}} \left (-\frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) & {\mathrm e}^{\frac {x}{2}} \left (\frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) \\ -{\mathrm e}^{-x} & {\mathrm e}^{\frac {x}{2}} \left (\frac {\cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}+\frac {\sqrt {3}\, \sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) & {\mathrm e}^{\frac {x}{2}} \left (\frac {\sqrt {3}\, \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{2}-\frac {\sin \left (\frac {\sqrt {3}\, x}{2}\right )}{2}\right ) \\ {\mathrm e}^{-x} & \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {x}{2}} & -\sin \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {x}{2}} \end {array}\right ]\cdot \left [\begin {array}{ccc} 1 & -\frac {1}{2} & \frac {\sqrt {3}}{2} \\ -1 & \frac {1}{2} & \frac {\sqrt {3}}{2} \\ 1 & 1 & 0 \end {array}\right ]^{-1} \\ {} & \circ & \textrm {Evaluate and simplify to get the fundamental matrix}\hspace {3pt} \\ {} & {} & \Phi \left (x \right )=\left [\begin {array}{ccc} \frac {2 \,{\mathrm e}^{\frac {3 x}{2}} {\mathrm e}^{-x} \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{3}+\frac {{\mathrm e}^{-x}}{3} & \frac {\left (\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} & -\frac {\left (-\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} \\ \frac {\left (-\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} & \frac {2 \,{\mathrm e}^{\frac {3 x}{2}} {\mathrm e}^{-x} \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{3}+\frac {{\mathrm e}^{-x}}{3} & \frac {\left (\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} \\ -\frac {\left (\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} & \frac {\left (-\sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-1\right ) {\mathrm e}^{-x}}{3} & \frac {2 \,{\mathrm e}^{\frac {3 x}{2}} {\mathrm e}^{-x} \cos \left (\frac {\sqrt {3}\, x}{2}\right )}{3}+\frac {{\mathrm e}^{-x}}{3} \end {array}\right ] \\ \square & {} & \textrm {Find a particular solution of the system of ODEs using variation of parameters}\hspace {3pt} \\ {} & \circ & \textrm {Let the particular solution be the fundamental matrix multiplied by}\hspace {3pt} {\moverset {\rightarrow }{v}}\left (x \right )\hspace {3pt}\textrm {and solve for}\hspace {3pt} {\moverset {\rightarrow }{v}}\left (x \right ) \\ {} & {} & {\moverset {\rightarrow }{y}}_{p}\left (x \right )=\Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right ) \\ {} & \circ & \textrm {Take the derivative of the particular solution}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}_{p}^{\prime }\left (x \right )=\Phi ^{\prime }\left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right )+\Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}^{\prime }\left (x \right ) \\ {} & \circ & \textrm {Substitute particular solution and its derivative into the system of ODEs}\hspace {3pt} \\ {} & {} & \Phi ^{\prime }\left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right )+\Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}^{\prime }\left (x \right )=A \cdot \Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right )+{\moverset {\rightarrow }{f}}\left (x \right ) \\ {} & \circ & \textrm {The fundamental matrix has columns that are solutions to the homogeneous system so its derivative follows that of the homogeneous system}\hspace {3pt} \\ {} & {} & A \cdot \Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right )+\Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}^{\prime }\left (x \right )=A \cdot \Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}\left (x \right )+{\moverset {\rightarrow }{f}}\left (x \right ) \\ {} & \circ & \textrm {Cancel like terms}\hspace {3pt} \\ {} & {} & \Phi \left (x \right )\cdot {\moverset {\rightarrow }{v}}^{\prime }\left (x \right )={\moverset {\rightarrow }{f}}\left (x \right ) \\ {} & \circ & \textrm {Multiply by the inverse of the fundamental matrix}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{v}}^{\prime }\left (x \right )=\Phi \left (x \right )^{-1}\cdot {\moverset {\rightarrow }{f}}\left (x \right ) \\ {} & \circ & \textrm {Integrate to solve for}\hspace {3pt} {\moverset {\rightarrow }{v}}\left (x \right ) \\ {} & {} & {\moverset {\rightarrow }{v}}\left (x \right )=\int _{0}^{x}\Phi \left (s \right )^{-1}\cdot {\moverset {\rightarrow }{f}}\left (s \right )d s \\ {} & \circ & \textrm {Plug}\hspace {3pt} {\moverset {\rightarrow }{v}}\left (x \right )\hspace {3pt}\textrm {into the equation for the particular solution}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}_{p}\left (x \right )=\Phi \left (x \right )\cdot \left (\int _{0}^{x}\Phi \left (s \right )^{-1}\cdot {\moverset {\rightarrow }{f}}\left (s \right )d s \right ) \\ {} & \circ & \textrm {Plug in the fundamental matrix and the forcing function and compute}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}_{p}\left (x \right )=\left [\begin {array}{c} -\frac {\left (-2 \sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+2 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-3 \,{\mathrm e}^{x} \cos \left (x \right )+3 \sin \left (x \right ) {\mathrm e}^{x}+1\right ) {\mathrm e}^{-x}}{6} \\ \frac {\left (2 \sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )-3 \,{\mathrm e}^{x} \cos \left (x \right )-3 \sin \left (x \right ) {\mathrm e}^{x}+2 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}+1\right ) {\mathrm e}^{-x}}{6} \\ -\frac {{\mathrm e}^{-x} \left (-\frac {4 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}}{3}+\frac {1}{3}+\left (\cos \left (x \right )-\sin \left (x \right )\right ) {\mathrm e}^{x}\right )}{2} \end {array}\right ] \\ \bullet & {} & \textrm {Plug particular solution back into general solution}\hspace {3pt} \\ {} & {} & {\moverset {\rightarrow }{y}}\left (x \right )=c_{1} {\moverset {\rightarrow }{y}}_{1}+c_{2} {\moverset {\rightarrow }{y}}_{2}\left (x \right )+c_{3} {\moverset {\rightarrow }{y}}_{3}\left (x \right )+\left [\begin {array}{c} -\frac {\left (-2 \sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+2 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}-3 \,{\mathrm e}^{x} \cos \left (x \right )+3 \sin \left (x \right ) {\mathrm e}^{x}+1\right ) {\mathrm e}^{-x}}{6} \\ \frac {\left (2 \sqrt {3}\, {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )-3 \,{\mathrm e}^{x} \cos \left (x \right )-3 \sin \left (x \right ) {\mathrm e}^{x}+2 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}+1\right ) {\mathrm e}^{-x}}{6} \\ -\frac {{\mathrm e}^{-x} \left (-\frac {4 \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}}{3}+\frac {1}{3}+\left (\cos \left (x \right )-\sin \left (x \right )\right ) {\mathrm e}^{x}\right )}{2} \end {array}\right ] \\ \bullet & {} & \textrm {First component of the vector is the solution to the ODE}\hspace {3pt} \\ {} & {} & y=-\frac {\left ({\mathrm e}^{\frac {3 x}{2}} \left (-\sqrt {3}\, c_{3} +c_{2} +\frac {2}{3}\right ) \cos \left (\frac {\sqrt {3}\, x}{2}\right )-\left (\left (c_{2} +\frac {2}{3}\right ) \sqrt {3}+c_{3} \right ) {\mathrm e}^{\frac {3 x}{2}} \sin \left (\frac {\sqrt {3}\, x}{2}\right )+\left (-\cos \left (x \right )+\sin \left (x \right )\right ) {\mathrm e}^{x}-2 c_{1} +\frac {1}{3}\right ) {\mathrm e}^{-x}}{2} \end {array} \]

Maple trace

`Methods for third order ODEs: 
--- Trying classification methods --- 
trying a quadrature 
trying high order exact linear fully integrable 
trying differential order: 3; linear nonhomogeneous with symmetry [0,1] 
trying high order linear exact nonhomogeneous 
trying differential order: 3; missing the dependent variable 
checking if the LODE has constant coefficients 
<- constant coefficients successful`
 

✓ Solution by Maple

Time used: 0.0 (sec). Leaf size: 48

dsolve(diff(y(x),x$3)+y(x)=cos(x),y(x), singsol=all)
 

\[ y \left (x \right ) = \left (\frac {\left (\cos \left (x \right )-\sin \left (x \right )\right ) {\mathrm e}^{x}}{2}+c_{3} \sin \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}+c_{2} \cos \left (\frac {\sqrt {3}\, x}{2}\right ) {\mathrm e}^{\frac {3 x}{2}}+c_{1} \right ) {\mathrm e}^{-x} \]

✓ Solution by Mathematica

Time used: 0.515 (sec). Leaf size: 68

DSolve[y'''[x]+y[x]==Cos[x],y[x],x,IncludeSingularSolutions -> True]
 

\[ y(x)\to -\frac {\sin (x)}{2}+\frac {\cos (x)}{2}+c_1 e^{-x}+c_3 e^{x/2} \cos \left (\frac {\sqrt {3} x}{2}\right )+c_2 e^{x/2} \sin \left (\frac {\sqrt {3} x}{2}\right ) \]