# Toronto Math Forum

## MAT244--2018F => MAT244--Tests => Quiz-4 => Topic started by: Victor Ivrii on October 26, 2018, 05:41:52 PM

Title: Q4 TUT 03014
Post by: Victor Ivrii on October 26, 2018, 05:41:52 PM
Find the general solution of the given differential equation.
$$y'' + 9y = 9 \sec^2 (3t),\qquad 0< t < \frac{\pi}{6}.$$
Title: Re: Q4 TUT 03014
Post by: Yunqi(Yuki) Huang on October 26, 2018, 05:46:15 PM
For the left-hand side, we could write as $r^2+9=0$.Thus $r=3i$ or $r=-3i.$
Now  $Y_{c}=C_{1}cos3t+C_{2}sin3t$
For the Right-hand side, $Y_{1}(t)=cos3t$, $Y_{2}(t)+sin3t$, $g(t)=9sec^2(3t)$
$$W(t)=\left[ \begin{matrix} cos3t & sin3t \\ -3sin3t& 3cos3t \end{matrix} \right] \tag{3}=3$$
$$W_{1}(t)=\left[ \begin{matrix} 0 & sin3t \\ 1 & 3cos3t \end{matrix} \right] \tag{3}=-sin3t$$
$$W_{2}(t)=\left[ \begin{matrix} cos3t & 0 \\ -3sin3t& 1 \end{matrix} \right] \tag{3}=cos3t$$
So, the particular solution is $$Y_{p}(t)=Y_{1}\int\frac{g(s)W_{1}(s)}{W(s)}\,ds+Y_{2}\int\frac{g(s)W_2(s)}{W(s)}\,ds=-1+\sin(3t)\ln |\sec(3t)+\tan(3t)|$$
Thus, the general solution is $Y(t)=C_{1}cos(3t)+C_{2}sin(3t)+sin(3t)ln$||sec(3t)+tan(3t)||-1
Title: Re: Q4 TUT 03014
Post by: Yunqi(Yuki) Huang on October 26, 2018, 05:59:28 PM
For the left-hand side, we could write as $r^2+9=0$.Thus $r=3i$ or $r=-3i.$
Now  $Y_{c}=C_{1}cos3t+C_{2}sin3t$
For the Right-hand side, $Y_{1}(t)=cos3t$$. Y_{2}(t)+sin3t$$. g(t)=9sec^2(3t)$
$$W(t)=\left[ \begin{matrix} cos3t & sin3t \\ -3sin3t& 3cos3t \end{matrix} \right] \tag{3}=3$$
$$W_{1}(t)=\left[ \begin{matrix} 0 & sin3t \\ 1 & 3cos3t \end{matrix} \right] \tag{3}=-sin3t$$
$$W_{2}(t)=\left[ \begin{matrix} cos3t & 0 \\ -3sin3t& 1 \end{matrix} \right] \tag{3}=cos3t$$
So, the particular solution is $Y_{p}(t)=Y_{1}\int\frac{g(s)W_{1}(s)}{W(s)}\,ds+Y_{2}\int\frac{W(s)}\,ds=-1+sin(3t)ln$||sec(3t)+tan(3t)||
Thus, the general solution is $Y(t)=C_{1}cos(3t)+C_{2}sin(3t)+sin(3t)ln$|sec(3t)+tan(3t)|-1
Title: Re: Q4 TUT 03014
Post by: Victor Ivrii on October 27, 2018, 12:35:27 PM
1) Need to escape \sin (x) , \ln (x) etc producing $\sin (x)$,...

2) Do not put dollar signs inside math formula! Only around it

Fix it

3) Where the constants from integration?
Title: Re: Q4 TUT 03014
Post by: Michael Poon on October 27, 2018, 03:57:36 PM
So, the particular solution is $Y_{p}(t)=Y_{1}\int\frac{g(s)W_{1}(s)}{W(s)}\,ds+Y_{2}\int\frac{W(s)}\,ds=-1+sin(3t)ln$||sec(3t)+tan(3t)||

Small correction, likely due to typesetting error, but I think you mean the following:

So, the particular solution is $Y_{p}(t)=Y_{1}\int\frac{g(s)W_{1}(s)}{W(s)}\,ds+Y_{2}\int\frac{g(s)W_{2}(s)}{W(s)}\,ds=-1+sin(3t)ln|sec(3t)+tan(3t)|$
Title: Re: Q4 TUT 03014
Post by: Victor Ivrii on October 27, 2018, 07:31:32 PM
Better but still not perfect

http://forum.math.toronto.edu/index.php?topic=1443.0 (http://forum.math.toronto.edu/index.php?topic=1443.0)

And what about constants (it should be a general solution)?
Title: Re: Q4 TUT 03014
Post by: Michael Poon on October 27, 2018, 09:07:57 PM
Right, I think it should be:

$Y(t) = Y_c + Y_p$

$Y(t) = c_1 \cos(3t) + c_2 \sin(3t)+ \sin(3t) \ln| \sec(3t)+ \tan(3t)|-1 + c_3$, where $c_1, c_2, c_3 \in {\rm I\!R}$

Since $c_3$ can absorb the "-1", it becomes:

$Y(t) = c_1 \cos(3t) + c_2 \sin(3t)+ \sin(3t) \ln| \sec(3t)+ \tan(3t)| + c_3$, where $c_1, c_2, c_3 \in {\rm I\!R}$
Title: Re: Q4 TUT 03014
Post by: Victor Ivrii on October 27, 2018, 11:42:03 PM
$c_3$? There are only 2 const, and $y=1$ is not a solution to homogeneous equation. And to make $\mathbb{R}$ (etc) you type \mathbb{R} !
Title: Re: Q4 TUT 03014
Post by: Yunqi(Yuki) Huang on October 28, 2018, 12:18:31 AM
For the left-hand side, we could write as $r^2+9=0$.Thus $r=3i$ or $r=-3i.$
Now  $Y_{c}=C_{1}cos3t+C_{2}sin3t$
For the Right-hand side, $Y_{1}(t)=cos3t$$. Y_{2}(t)+sin3t$$. g(t)=9sec^2(3t)$
$$W(t)=\left[ \begin{matrix} cos3t & sin3t \\ -3sin3t& 3cos3t \end{matrix} \right] \tag{3}=3$$
$$W_{1}(t)=\left[ \begin{matrix} 0 & sin3t \\ 1 & 3cos3t \end{matrix} \right] \tag{3}=-sin3t$$
$$W_{2}(t)=\left[ \begin{matrix} cos3t & 0 \\ -3sin3t& 1 \end{matrix} \right] \tag{3}=cos3t$$
We could check the integral table that $\int\tan(t)\sec(t)dt=sec(t)$. $\int\sec(t)dt=ln|sec(t)+tan(t)|$
So, the particular solution is $Y_{p}(t)=Y_{1}\int\frac{g(s)W_{1}(s)}{W(s)}\,ds+Y_{2}\int\frac{g(s)W_{2}(s)}{W(s)}\,ds=-3\cos(3t)\int\tan(3s)\sec(3s)ds+3\sin(3t)\int\sec(3t)ds=-3\cos3t*\frac{1}{3}sec(3t)+3\sin(3t)ln|\sec3t+tan3t|*\frac{1}{3}$
Thus, the general solution is $Y(t)=C_{1}\cos(3t)+C_{2}\sin(3t)+\sin(3t)ln|\sec(3t)+\tan(3t)|-1$