# Toronto Math Forum

## MAT244-2013F => MAT244 Math--Tests => Quiz 3 => Topic started by: Victor Ivrii on November 06, 2013, 08:12:36 PM

Title: Problem 2 (night sections)
Post by: Victor Ivrii on November 06, 2013, 08:12:36 PM
Find the general solution of the given differential equation. Leave your answer in terms of one or more integrals.
\begin{equation*}
y'''-y'' + y'-y = \sec (t), \qquad -\frac{\pi}{2} < t < \frac{\pi}{2}.
\end{equation*}
Title: Re: Problem 2 (night sections)
Post by: Ka Hou Cheok on November 06, 2013, 08:38:09 PM
\end{equation*}

The responding characteristic equation is $$r^3-r^2+r-1=0$$ and we get $r_1=1, r_2=i, r_3=-i$. So $$y_c=c_1e^t+c_2\cos(t)+c_3\sin(t)$$

$$W=e^t((\sin^2(t)+\cos^2(t)-\sin(t)\cos(t))-(-\sin^2(t)-\cos^2(t)-\sin(t)\cos(t)))=2e^t$$
$$W_1=\cos^2(t)+\sin^2(t)=1\\ W_2=e^t(\sin(t)-\cos(t))\\ W_3=e^t(-\sin(t)-\cos(t))$$

$$u_1=\int \frac{(\sec(t))(1)}{2e^t}dt\\ u_2=\int \frac{(\sec(t))(e^t(\sin(t)-\cos(t))}{2e^t}dt\\ u_3=\int \frac{(\sec(t))(e^t(-\sin(t)-\cos(t))}{2e^t}dt$$

$$y=y_c+y_1u_1+y_2u_2+y_3u_3 =c_1e^t+c_2\cos(t)+c_3\sin(t)+\\ e^t\int \frac{(\sec(t))(1)}{2e^t}dt+ \cos(t)\int \frac{(\sec(t))(e^t(\sin(t)-\cos(t))}{2e^t}dt+ \sin(t)\int \frac{(\sec(t))(e^t(-\sin(t)-\cos(t))}{2e^t}dt$$

As the question stated my answer can be in terms of one or more integrals, hopefully I can stop here.
Title: Re: Problem 2 (night sections)
Post by: Yangming Cai on November 06, 2013, 09:08:39 PM
I am so impressed with your speed.
Title: Re: Problem 2 (night sections)
Post by: Ka Hou Cheok on November 06, 2013, 09:51:57 PM
I am so impressed with your speed.

I would take it as a compliment. Thanks.
I'm impressed and appreciate your results of the integrals. I was too lazy to integrate them.
Title: Re: Problem 2 (night sections)
Post by: Victor Ivrii on November 07, 2013, 05:11:01 AM
I replaced
Code: [Select]
 sin , cos , sec by
Code: [Select]
\sin, \cos, \sec and keyboard sign of integral by
Code: [Select]
\int
Title: Re: Problem 2 (night sections)
Post by: Ka Hou Cheok on November 07, 2013, 09:42:05 AM
I replaced
Code: [Select]
 sin , cos , sec by
Code: [Select]
\sin, \cos, \sec and keyboard sign of integral by
Code: [Select]
\int

Thanks, Prof Victor. I'll use them well next time.