Posted by Vincent on Saturday, December 6, 2008 at 11:43pm.
please help me with this proving question
prove that
4cos(theta) * cos(theta + 120) * cos(theta  120) = cos(3theta)
PLEASE HELP THANKS

math  drwls, Sunday, December 7, 2008 at 1:09am
Use the cos (A+B) and cos (AB) formulas to rewrite the left hand side in terms of sin theta and cost theta.
The product (cos(A+B)*cos(AB) can be written
(cos A cos B)^2  (sinA sinB)^2
which when B = 120 is
(1/4)cos^2A (3/4)sin^2A
= (1/4)cos^2A (3/4)(1  cos^2A)
= cos^2A  (3/4)
So the left side becomes
4 cos^3(theta)3 cos theta
Now you work on the right hand side, treating cos (3 theta) as cos (theta + 2 theta) and provde it equals the left hand side.

math  Vincent, Sunday, December 7, 2008 at 1:32am
damn thanks

math  drwls, Sunday, December 7, 2008 at 9:24am
I converted the more difficult half of the equation for you. The answer is correct. You should have very little trouble proving the right side is the same.
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