# Find all solutions

• May 15th 2010, 09:03 AM
npthardcorebmore
Find all solutions
Find all solutions for the following ODE:

\$\displaystyle y''+w^2y=Acos(wx), A /= 0 , w>0\$

I figured it might be easier to solve as:
\$\displaystyle
y''+w^2y-Acos(wx)=0\$

but that still has me somewhat confused.(Headbang)
• May 15th 2010, 02:34 PM
HallsofIvy
Quote:

Originally Posted by npthardcorebmore
Find all solutions for the following ODE:

\$\displaystyle y''+w^2y=Acos(wx), A /= 0 , w>0\$

I figured it might be easier to solve as:
\$\displaystyle
y''+w^2y-Acos(wx)=0\$

but that still has me somewhat confused.(Headbang)

Do it as two parts- first solve the "corresponding homogeneous equation" which is y"+ w^2y= 0. That has characteristic equation \$\displaystyle r^2+ w^2= 0\$ which has roots iw and -iw. Do you know how to construct the general solution to the homogeneous differential equation from that?

Now, search for a single solution to the entire equation. Normally, I would suggest something of the form y= A cos(wx)+ B sin(wx) but after you have solved the corresponding homogeneous equation, you may see why you should try y= A x cos(wx)+ B x sin(wx) instead. Find y" for that, put into the equation and solve for A and B.

The general solution to the entire equation is the sum of the general solution to the corresponding homogeneous equation and any single solution to the entire equation.