# Finding General Solution and Initial Value Problem for Differential equation

• Apr 23rd 2014, 09:09 PM
grandmarquis84
Finding General Solution and Initial Value Problem for Differential equation
Hi, I have the equation y''+100y = 0.5cos(10t) and I have to find the general solution and the IVP.

For the general solution I made the y's into s's and figured out s^2 + 100 = 0 so S 1 & 2 = 0 +/- 10i

From here I found the y-homogeneous part = k1(cos(10t))+k2(sin(10t))

so y-particular = Acos(10t), and I found the second derivitive to equal -100Acos(10t)

The problem is I don't know what to do next. If I plug the A's back in I get -100A+100A = 0.5 and I know this cannot be
• Apr 23rd 2014, 10:16 PM
Prove It
Re: Finding General Solution and Initial Value Problem for Differential equation
Since the RHS of the DE is a part of the homogeneous solution, for a particular solution you should actually try \displaystyle \begin{align*} y_p = A\,x\cos{(10t)} + B\,x\sin{(10t)} \end{align*}.
• Apr 24th 2014, 05:29 AM
HallsofIvy
Re: Finding General Solution and Initial Value Problem for Differential equation
Quote:

Originally Posted by Prove It
Since the RHS of the DE is a part of the homogeneous solution, for a particular solution you should actually try \displaystyle \begin{align*} y_p = A\,x\cos{(10t)} + B\,x\sin{(10t)} \end{align*}.

Typo: obviously, this should be $\displaystyle y_p= At cos(10t)+ Bt sin(10t)$.
• Apr 24th 2014, 08:57 AM
Prove It
Re: Finding General Solution and Initial Value Problem for Differential equation
Quote:

Originally Posted by HallsofIvy
Typo: obviously, this should be $\displaystyle y_p= At cos(10t)+ Bt sin(10t)$.

Yes, (Swear) these interchangeable letters they have for independent variables :P hahaha.