Question

Can somebody show me how to solve cos(x)+cos(2x) = -1

I know the answer is (pi/2) but when I tried to solve it, I got (pi/3), and I'm not sure how to do it right.

Answer 1

you can expand cos(2x)

Answer 2

cos(2x)
= cosx * cos x - sin x * sin x
= cos^2(x) - sin^2(x)

substitute 1-cos^2(x) for sin^2(x)

= cos^2(x) - (1 - cos^2(x)

Answer 3

Ah, ok. Then solve it like a quadratic?

Answer 4

correct

Answer 5

Ok great. Thanks a lot.

Answer 6

yes good work men

Answer 7

dan :D

Answer 8

perl :D

Answer 9

baby:d

Answer 10

hey batman

Answer 11

Congrats on 99

Answer 12

thanks :)