1. ## General solution

Find the general solution, in radians, of the equation

cos2a + cos4a + cos6a = 0

I have this equation to play with:

a = 2πn (+or-) α

but i dont know what to do with it :S

2. Originally Posted by djmccabie
Find the general solution, in radians, of the equation

cos2a + cos4a + cos6a = 0

I have this equation to play with:

a = 2πn (+or-) α

but i dont know what to do with it :S
Hint: $\cos(2a) + \cos(6a)=2 \cos(2a) \cos (4a).$

3. Why is this true :S

4. Originally Posted by djmccabie
Why is this true :S
in general we have this identity: $\cos x + \cos y = 2 \cos \left(\frac{x+y}{2} \right) \cos \left(\frac{x-y}{2} \right).$ do you remember? now just put $x=6a$ and $y=2a$ and see what you get!

5. Originally Posted by NonCommAlg
in general we have this identity: $\cos x + \cos y = 2 \cos \left(\frac{x+y}{2} \right) \cos \left(\frac{x-y}{2} \right).$ do you remember?
No. I gotta teach myself a lot of the maths I haven't come across this thanks a lot!

are there any more important ones i should be aware of? I have learned compound angle and double angle formulae