91Ó°ÊÓ

Elimination Procedure for 2 × 2 Systems

To find a general solution for the system

$$\begin{gathered} {\mathbf{L}}_{\mathbf{1}}\left[\mathbf{x}\right]\mathbf{+}{\mathbf{L}}_{\mathbf{2}}\left[\mathbf{y}\right]\mathbf{=}{\mathbf{f}}_{\mathbf{1}}\mathbf{,} \\ {\mathbf{L}}_{\mathbf{3}}\left[\mathbf{x}\right]\mathbf{+}{\mathbf{L}}_{\mathbf{4}}\left[\mathbf{y}\right]\mathbf{=}{\mathbf{f}}_{\mathbf{2}}\mathbf{,} \end{gathered}$$

Where${\mathbf{L}}_{\mathbf{1}}\mathbf{,}{\mathbf{L}}_{\mathbf{2}}\mathbf{,}{\mathbf{L}}_{\mathbf{3}}\mathbf{,}$ and L₄are polynomials in$D=\frac{d}{dt}$

1. Make sure that the system is written in operator form.
2. Eliminate one of the variables, say, y, and solve the resulting equation for x(t). If the system is degenerating, stop! A separate analysis is required to determine whether or not there are solutions.
3. (Shortcut) If possible, use the system to derive an equation that involves y(t) but not its derivatives. [Otherwise, go to step (d).] Substitute the found expression for x(t) into this equation to get a formula for y(t). The expressions for x(t), and y(t) give the desired general solution.
4. Eliminate x from the system and solve for y(t). [Solving for y(t) gives more constants----in fact, twice as many as needed.]
5. Remove the extra constants by substituting the expressions for x(t) and y(t) into one or both of the equations in the system. Write the expressions for x(t) and y(t) in terms of the remaining constants.

Given that,

$x\text{'}+y\text{'}+2x=0           ......\mathbf{\left(}\mathbf{1}\mathbf{\right)}$

$x\text{'}+y\text{'}-x-y=\sin t     ......\mathbf{\left(}\mathbf{2}\mathbf{\right)}$

Let us rewrite this system of operators in operator form:

$$\begin{gathered} \left(D+2\right)\left[x\right]+D\left[y\right]=0                  ......\mathbf{\left(}\mathbf{3}\mathbf{\right)} \\ \left(D-1\right)\left[x\right]+\left(D-1\right)\left[y\right]=\sin t            ......\mathbf{\left(}\mathbf{4}\mathbf{\right)} \end{gathered}$$

Multiply (D-1) on equation (3).

$\left(D-1\right)\left(D+2\right)\left[x\right]+D\left(D-1\right)\left[y\right]=0        ......\mathbf{\left(}\mathbf{5}\mathbf{\right)}$

And multiply -D on equation (2).

$$\begin{gathered} -D\left(D-1\right)\left[x\right]-D\left(D-1\right)\left[y\right]=-D\left[\sin t\right] \\ \left(-D^2+D\right)\left[x\right]-D\left(D-1\right)\left[y\right]=-\cos t           ......\mathbf{\left(}\mathbf{6}\mathbf{\right)} \end{gathered}$$

Then add equations (3) and (4) together one gets,

$$\begin{gathered} \left(D-1\right)\left(D+2\right)\left[x\right]+\left(-D^2+D\right)\left[x\right]=0-\cos t \\ \left(D^2+D-2-D^2+D\right)\left[x\right]=-\cos t \\ \left(2D-2\right)\left[x\right]=-\cos t \\ \left(2D-2\right)\left[x\right]=-\cos t             ......\mathbf{\left(}\mathbf{7}\mathbf{\right)} \end{gathered}$$

Since the auxiliary equation to the corresponding homogeneous equation is 2r-2=0. The root is r=1.

Then, the homogeneous solution of u is$x_h\left(t\right)=c_1{e}^t        ......\mathbf{\left(}\mathbf{8}\mathbf{\right)}$

Let us take the undetermined coefficients and assume that

$x_p\left(t\right)=A\cos t+B\sin t      ......\mathbf{\left(}\mathbf{9}\mathbf{\right)}$

Now derivate the equation (7)

$D\left[x_p\left(t\right)\right]=-A\sin t+B\cos t$

Substitute the derivative in equation (5).

$$\begin{gathered} \left(2D-2\right)\left[A\cos t+B\sin t\right]=-\cos t \\ -2A\sin t+2B\cos t-2A\cos t-2B\sin t=-\cos t \\ \left(-2A+2B\right)\cos t-2\left(A+B\right)\sin t=-\cos t \end{gathered}$$

Now, equalize the like terms.

$$\begin{gathered} -2A+2B=-1 \\ -2A-2B=0 \end{gathered}$$

Solve the equations to find the value of A and B.

$$\begin{gathered} -4A=-1 \\ A=\frac{1}{4} \end{gathered}$$

Then,

$$\begin{gathered} -2\left(\frac{1}{4}\right)-2B=0 \\ -2B=\frac{1}{2} \\ B=-\frac{1}{4} \end{gathered}$$

So,$x_p\left(t\right)=\frac{1}{4}\cos t-\frac{1}{4}\sin t$

Use equations (6) and (8) to get,

$$\begin{gathered} x\left(t\right)=x_h\left(t\right)+x_p\left(t\right) \\ x\left(t\right)=c_1{e}^t+\frac{1}{4}\cos t-\frac{1}{4}\sin t \end{gathered}$$

Subtract the equation (3) and (4).

$$\begin{gathered} \left(D+2\right)\left[x\right]+D\left[y\right]-\left(\left(D-1\right)\left[x\right]+\left(D-1\right)\left[y\right]\right)=0-\sin t \\ \left(D+2-D+1\right)\left[x\right]+\left(D-D+1\right)\left[y\right]=-\sin t \\ 3\left[x\right]+\left[y\right]=-\sin t \\ \left[y\right]=-\sin t-3\left[x\right] \\ =-\sin t-3\left[c_1{e}^t+\frac{1}{4}\cos t-\frac{1}{4}\sin t\right] \\ =-3c_1{e}^t-\frac{3}{4}\cos t+\frac{3}{4}\sin t-\sin t \\ y\left(t\right)=-3c_1{e}^t-\frac{3}{4}\cos t-\frac{1}{4}\sin t \end{gathered}$$

So, the solution is founded