show that cos 3(theta) = 4cos^3(theta)-3cos(theta)
where do i even start? :o
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show that cos 3(theta) = 4cos^3(theta)-3cos(theta)
where do i even start? :o
Is this want you would like to prove?
$\displaystyle cos (3\theta) = 4cos^3(\theta)-3cos(\theta)$
yess... thanks for making it the way it should! :)
$\displaystyle \cos{3\theta}\ =\ \cos{(2\theta+\theta)}$Quote:
Originally Posted by ohhhh
Now apply the formula $\displaystyle \cos{(A+B)}=\cos{A}\cos{B}-\sin{A}\sin{B}$ as well as the double-angle formula for sine and for cosine.
ok, so let me see.
cos (2a+a) = cos2(a)*cos(a) - sin2(a)*sin(a)
= cos^2a-sin^2a - 2sin(a)*cos(a)*sin(a)
= cos^2a-sin^2a - 2sin^2(a)*cos(a)
...is this right so far?
Complex numbers also work:Quote:
Originally Posted by ohhhh
$\displaystyle \cos 3\theta = \text{Re} \left( {\cos \theta + i\sin \theta} \right)^3 = \text{Re} \left( {\cos ^3 \theta + 3i\sin \theta\cos ^2 \theta - 3\sin ^2 \theta\cos \theta - i\sin ^3 \theta} \right).$
So $\displaystyle \cos 3\theta = \cos ^3 \theta - 3\cos \theta(1 - \cos ^2 \theta) = 4\cos ^3 \theta - 3\cos \theta\,\blacksquare$
thanks a lot for your help guys. got it down! :)
rightQuote:
cos (2a+a) = cos2(a)*cos(a) - sin2(a)*sin(a)
You have lost a cos(a). It should readQuote:
= cos^2a-sin^2a - 2sin(a)*cos(a)*sin(a)
$\displaystyle \cos (a)(\cos ^2(a)-\sin ^2(a))-2\sin (a) \cos (a) \sin (a)$
