# Trig identities

• November 2nd 2008, 10:12 PM
ssmh
Trig identities
Cos(2x)=-(√2)/2

Can anyone help me solve this?
• November 2nd 2008, 11:07 PM
11rdc11
Quote:

Originally Posted by ssmh
Cos(2x)=-(√2)/2

Can anyone help me solve this?

I kind of sleepy so hopefully I don't make a mistake lol

$\cos{(2x)}=-\frac{\sqrt{2}}{2}$

Ok so you know cos is negative in the 2nd and 3rd quadrant.

Also $\cos{x} = -\frac{\sqrt{2}}{2}$ at 135 degrees and 225

Now just divide 135 and 225 by 2 and it will give you your answer

$67.5 + 360n$

$112.5 + 360n$

$n = 0,\pm 1, \pm 2,...$