Having trouble doing this differential equation ...

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Sicar

Joined: 20 Sep 2009
Posts: 3

21 Sep 2009 23:00 Having trouble doing this differential equation ...

First time taking this course, concept is lost on me. Problem seems simple enough.

x" + 4x = 0

x(0) = 1 x(0)'=0 given initial values ..

I just don't know where to start with it. It needs to be done without using LaPlace transforms?

_Eduardo_

Joined: 31 Aug 2009
Posts: 5
Helped: 1
Location: Argentina

22 Sep 2009 1:09 Re: Having trouble doing this differential equation ...

x" + 4x = 0

x(0) = 1 x(0)'=0 given initial values ..

You must know some theory of differential equations:
http://en.wikipedia.org/wiki/Linear_differential_equation

Then, the characteristic equation is z^2+4 = 0 --> z = +/- 2j (j imaginary unit)

The general solution is:
x(t) = A*exp(j 2t) + B*exp(-j 2t) = C*cos(2t) + D*sin(2t)

But x(0) = 1 and x(0)'=0 --> C=1, D=0 --> x(t) = cos(2t)