# Thread: Using complex numbers for 2nd order DE

1. ## Using complex numbers for 2nd order DE

I can change this equation, using Eulers method, and get the answers using $sin$ and $cos$, but using complex numbers is apparently faster and we have to do it this way for my professor...so if you could help me figure this out I would appreciate it.

$y''-8y'+25y=3 e^{4x} cos(3x)$

HERE IS WHAT I DID:
First, I'll solve the homogeneous case to find the complementary solution and the roots.
$r^{2}-8r+25=0$
$r=4+3i$ or $r=4-3i$
$y_{c}=c_{1} e^{4x}cos(3x)+c_{2}e^{4x}sin(3x)$

Now, to find the solution $y=y_{c}+y_{p}$, where $y_{p}$ is the particular solution to the non-homogeneous equation, I need help. I guessed a few functions but none seemed to work out.

Here is what Ive tried:
Since $3e^{4x}cos(3x)=3e^{(4+3i)x}$...well the real part I think
and
$4+3i$ is a root of the homogenous equatoin, so I need multiply my guess for $y_{p}$ by $x$

From here I cannot figure out what function I need to use...

2. Originally Posted by snaes
I can change this equation, using Eulers method, and get the answers using $sin$ and $cos$, but using complex numbers is apparently faster and we have to do it this way for my professor...so if you could help me figure this out I would appreciate it.

$y''-8y'+25y=3 e^{4x} cos(3x)$

HERE IS WHAT I DID:
First, I'll solve the homogeneous case to find the complementary solution and the roots.
$r^{2}-8r+25=0$
$r=4+3i$ or $r=4-3i$
$y_{c}=c_{1} e^{4x}cos(3x)+c_{2}e^{4x}sin(3x)$

Now, to find the solution $y=y_{c}+y_{p}$, where $y_{p}$ is the particular solution to the non-homogeneous equation, I need help. I guessed a few functions but none seemed to work out.

Here is what Ive tried:
Since $3e^{4x}cos(3x)=3e^{(4+3i)x}$...well the real part I think
and
$4+3i$ is a root of the homogenous equatoin, so I need multiply my guess for $y_{p}$ by $x$

From here I cannot figure out what function I need to use...
Since the complementary solution involves the RHS, you will need to guess a particular solution of the form

$y_p = A\color{red}x\color{black}e^{4x}\cos{3x} + B\color{red}x\color{black}e^{4x}\sin{3x}$.

3. Thanks, but is there any way to convert this into a "complex" valued expression. I need to use something like: $a x e^{(4+3i)x}$.

 for myhomework we are supposed to do each problem with real valued (sine and cosine) which i can do AND using some $e^{a+bi}$ kind of form, which i cannot do. Any help would be great.