Question

If cos(x) = 2/3, then the value of cos(2x) is
a) -1/9 b) -5/9 c) 5/9 d) 1/9

Is it (a) or (d)?

Answer 1

\[2\cos^2(x)-1\]

Answer 2

so
\[2(\frac{2}{3})^2-1=2\times \frac{4}{9}-1=\frac{8}{9}-1=-\frac{1}{9}\]

Answer 3

Great! Thanks!
That's what I was thinking, I just subtracted wrong.