# Need help with a double angle question

• Nov 18th 2012, 12:39 PM
SomeGuy23
Need help with a double angle question
If cos2x=12/13 and 2x is in the first quadrant, evaluate the six trigonometric functions for x, without your calculator. The answer is:

cos x= 5/√26; sec x=√26/5

sinx=1/
√26; csc x=√26

tanx=1/5; cot x=5

How do I get these answers? Any help would be greatly appreciated.
• Nov 18th 2012, 12:54 PM
HallsofIvy
Re: Need help with a double angle question
Do you know either the "double angle formulas" or the "half angle formulas"?
• Nov 18th 2012, 12:57 PM
skeeter
Re: Need help with a double angle question
since $0 < 2x < \frac{\pi}{2}$ , then $0 < x < \frac{\pi}{4}$

$\cos{x} = \sqrt{\frac{1 + \cos(2x)}{2}}$

$\sin{x} = \sqrt{\frac{1 - \cos(2x)}{2}}$

sub in the value given for $\cos(2x)$ ...
• Nov 18th 2012, 02:24 PM
SomeGuy23
Re: Need help with a double angle question
Thanks guys, I got it now.