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 (A-B) formulas to rewrite the left hand side in terms of sin theta and cost theta.

The product (cos(A+B)*cos(A-B) 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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