1. ## Solving for x

This is part of a bigger question to do with polar coordinates - I have obtained a derivative and equated it to zero in order to indicate a stationary point and this is what I need to solve:

$\displaystyle 2cos(2x)cos(x) - sin(2x)sin(x) = 0$

Which of the double/half/compound angle formulas do I need to use here? I always seem to forget in questions like these.

Thanks if you can help me :]

2. Originally Posted by db5vry
This is part of a bigger question to do with polar coordinates - I have obtained a derivative and equated it to zero in order to indicate a stationary point and this is what I need to solve:

$\displaystyle 2cos(2x)cos(x) - sin(2x)sin(x) = 0$

Which of the double/half/compound angle formulas do I need to use here? I always seem to forget in questions like these.

Thanks if you can help me :]
Dear db5vry,

Try, $\displaystyle sin2x=2sinxcosx~and~cos2x=2cos^{2}x-1=1-2sin^{2}x=cos^{2}x-sin^{2}x$

3. Originally Posted by Sudharaka
Dear db5vry,

Try, $\displaystyle sin2x=2sinxcosx~and~cos2x=2cos^{2}x-1=1-2sin^{2}x=cos^{2}x-sin^{2}x$

$\displaystyle cosx (6sin^2x - 2) = 0$ which gives the solutions $\displaystyle \frac{\pi}{2}$ and $\displaystyle \frac{\sqrt{3}}{3}$? If so then I've done this correctly, but I'm not sure!
$\displaystyle cosx (6sin^2x - 2) = 0$ which gives the solutions $\displaystyle \frac{\pi}{2}$ and $\displaystyle \frac{\sqrt{3}}{3}$? If so then I've done this correctly, but I'm not sure!