Laplace

[TSN79]

Does anyone know how to convert

cos^2 (2t)

into a form that I can use the Laplace-table on...?

[Tide]

How about using some trig identities:

\cos^2 2x = \frac {1 + \cos 4x}{2}

[TSN79]

Hey thanks Tide! Just one thing, I wasn't really able to find this identity anywhere in my books, and I'm not really at a level where I can come up with such identities on my own if it goes beyond turning equations around. This identity is not one of the most used is it?

[Tide]

TSN,

It's just a variant of the sum formula which is very commonly used:

\cos a + b = \cos a \cos b - \sin a \sin b

so that when a = b

\cos 2a = \cos^2 a - \sin^2 a

and since

\sin^2 a + \cos^2 a = 1

the identity becomes

\cos 2a = 2 \cos^2 a - 1

from which

\cos^2 a = \frac {1 + \cos 2a}{2}

Finally, just set a = 2x for your problem.

[Astronuc]

For trigonometric identities, try:

http://mathworld.wolfram.com/TrigonometricAdditionFormulas.html