rework cos 2?: cos2? = cos^2 ? - sin^2 ? = 2 cos ^2 ? - a million sparkling up: 2 cos^2 ? - a million - cos ? = 0, use quadratic formula. inspite of each little thing that artwork, you will locate that one answer is 0. See what the different answer is, whether this is valid.

I assume you mean cosΘ= - (root3)/2

cos (a + b) = cos a*cos b -sin a* sin b

cos (2*z) = cos^2z - sin^2 z

= (3/4) - (1/4) = 1/2 <<<

cos 2Ө = cos ² Ө - sin ² Ө

cos 2Ө = 3/4 - 1/4 = 1/2

you would solve this with math

1. identify the problem

2. panic

3. try to figure it out

4. give up

5. make it look like you tried to figure it out

6. tell your teacher you tried but couldnt do it