91Ó°ÊÓ

The given equation is;

$\mathrm{y}\text{'}\text{'}+9\mathrm{y}=4{\mathrm{t}}^3\sin 3\mathrm{t}\text{ â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰}.....\left(1\right)$

According to the method of undetermined coefficients, to find a particular solution to the differential equation

$\mathrm{ay}\text{'}\text{'}+\mathrm{by}\text{'}+\mathrm{cy}=\left\{\begin{array}{l}{\mathrm{Ct}}^{\mathrm{m}}{\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{³¦´Ç²õβ³Ù}\\ \text{ â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰}\mathrm{or}\\ {\mathrm{Ct}}^{\mathrm{m}}{\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{²õ¾\pm ²Ôβ³Ù}\end{array}\right\}$

For,$\mathbf{β}≠\mathbf{0}$ use the form

${\mathrm{y}}_{\mathrm{p}}\left(\mathrm{x}\right)={\mathrm{t}}^{\mathrm{s}}[({\mathrm{A}}_{\mathrm{m}}{\mathrm{t}}^{\mathrm{m}}+...+{\mathrm{A}}_1\mathrm{t}+{\mathrm{A}}_0){\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{³¦´Ç²õβ³Ù}+{\mathrm{t}}^{\mathrm{s}}({\mathrm{B}}_{\mathrm{m}}{\mathrm{t}}^{\mathrm{m}}+...+{\mathrm{B}}_1\mathrm{t}+{\mathrm{B}}_0){\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{²õ¾\pm ²Ôβ³Ù}]$

With s = 1 , $\mathbf{Î\pm }\mathbf{+}\mathbf{¾\pm β}$if is a root of the associated auxiliary equation.

Write the homogeneous differential equation of the equation (1),

$\mathbf{y}\mathbf{\text{'}}\mathbf{\text{'}}\mathbf{+}\mathbf{9}\mathbf{y}\mathbf{=}\mathbf{0}$

The auxiliary equation for the above equation,

${\mathbf{m}}^2\mathbf{+}\mathbf{9}\mathbf{=}\mathbf{0}$

Solve the auxiliary equation,

$\begin{array}{c}{\mathrm{m}}^2+9=0\\ \mathrm{m}=Â\pm 3\mathrm{i}\end{array}$

The roots of the auxiliary equation are,

${\mathrm{m}}_1=3\mathrm{i},\text{ â¶Ä‰â¶Ä‰}\&\text{ â¶Ä‰â¶Ä‰}{\mathrm{m}}_2=-3\mathrm{i}$

The complementary solution of the given equation is,

${\mathrm{y}}_{\mathrm{c}}={\mathrm{c}}_1\cos 3\mathrm{x}+{\mathrm{c}}_2\sin 3\mathrm{x}$

To find a particular solution to the differential equation

$\mathrm{ay}\text{'}\text{'}+\mathrm{by}\text{'}+\mathrm{cy}=\left\{\begin{array}{l}{\mathrm{Ct}}^{\mathrm{m}}{\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{³¦´Ç²õβ³Ù}\\ \text{ â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰â¶Ä‰}\mathrm{or}\\ {\mathrm{Ct}}^{\mathrm{m}}{\mathrm{e}}^{\mathrm{Î\pm ³Ù}}\mathrm{²õ¾\pm ²Ôβ³Ù}\end{array}\right\}$

Compare with the given differential equation,

$\mathrm{y}\text{'}\text{'}+9\mathrm{y}=4{\mathrm{t}}^3\sin 3\mathrm{t}$

We have,

$\mathrm{Î\pm }=0,\text{ â¶Ä‰}\mathrm{β}=3$

Therefore, one gets

S = 1

And

$\begin{array}{c}\mathrm{Î\pm }+\mathrm{¾\pm β}=0+3\mathrm{i}\\ ={\mathrm{m}}_1\end{array}$

The particular solution to the differential equation for m = 3,

$\begin{array}{c}{\mathrm{y}}_{\mathrm{p}}\left(\mathrm{x}\right)=\mathrm{t}[({\mathrm{A}}_3{\mathrm{t}}^3+{\mathrm{A}}_2{\mathrm{t}}^2+{\mathrm{A}}_1\mathrm{t}+{\mathrm{A}}_0)\cos 3\mathrm{t}+({\mathrm{B}}_3{\mathrm{t}}^3+{\mathrm{B}}_2{\mathrm{t}}^2+{\mathrm{B}}_1\mathrm{t}+{\mathrm{B}}_0)\sin 3\mathrm{t}]\\ {\mathrm{y}}_{\mathrm{p}}\left(\mathrm{x}\right)=({\mathrm{A}}_3{\mathrm{t}}^4+{\mathrm{A}}_2{\mathrm{t}}^3+{\mathrm{A}}_1{\mathrm{t}}^2+{\mathrm{A}}_0\mathrm{t})\cos 3\mathrm{t}+({\mathrm{B}}_3{\mathrm{t}}^4+{\mathrm{B}}_2{\mathrm{t}}^3+{\mathrm{B}}_1{\mathrm{t}}^2+{\mathrm{B}}_0\mathrm{t})\sin 3\mathrm{t}\end{array}$