# Trig Equation

• October 2nd 2008, 05:36 AM
xwrathbringerx
Trig Equation
Can anyone please show me how to do 2cos2x=7cosx. I have no clue.

• October 2nd 2008, 05:52 AM
mr fantastic
Quote:

Originally Posted by xwrathbringerx
Can anyone please show me how to do 2cos2x=7cosx. I have no clue.

$2 \left( 2 \cos^2 (x) - 1 \right) = 7 \cos (x)$.

Now re-arrange into a quadratic in cos x .....
• October 2nd 2008, 11:32 AM
dolphinlover
cos2x=2(cos^2)x-1 (double angle)...that is how Mr Fantastic got his result.

Rearranging into a quadratic in cos x should yield...

(4cosx +1)(cosx -2) = 0

(4cosx) + 1 = 0 and
(cosx) - 2 = 0

....now solve for x in both