# Small trigonometry question

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• Mar 11th 2010, 02:42 PM
db5vry
Small trigonometry question
How would I go about solving the equation

cos x = cos 3x ?

I know the equation for cos 2x but this doesn't help - I get $2cos^2(3x/2) - 1$ which doesn't really help. Is there any other way which it can be done?
• Mar 11th 2010, 02:52 PM
Archie Meade
Quote:

Originally Posted by db5vry
How would I go about solving the equation

cos x = cos 3x ?

I know the equation for cos 2x but this doesn't help - I get $2cos^2(3x/2) - 1$ which doesn't really help. Is there any other way which it can be done?

Sure db5vry,

express Cos(3x) as Cos(2x+x)

expand this using Cos(A+B) and expand Cos(2x) later using Cos(x+x).
• Mar 11th 2010, 02:59 PM
Krizalid
or turn $\cos x-\cos3x$ by using sum - product formulae.