which cofunction identities would u use

cos (x+pi/2)=-sinx or sin (x+pi/2)=cosx? how did u determine which one to use?

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- Apr 6th 2009, 11:27 AMjepal[SOLVED] Given sin (pi/6)=1/2 use an equivalent trig expression to show cos 2pi/3=-1/
which cofunction identities would u use

cos (x+pi/2)=-sinx or sin (x+pi/2)=cosx? how did u determine which one to use? - Apr 6th 2009, 11:58 AMstapel
- Apr 6th 2009, 12:00 PMjepal
- Apr 6th 2009, 12:29 PMicemanfan
$\displaystyle \sin (\frac{\pi}{6}) = \frac{1}{2}$

$\displaystyle \sin (\theta) = \cos (\frac{\pi}{2} - \theta)$

Hence, $\displaystyle \cos (\frac{\pi}{3}) = \frac{1}{2}$

Also, $\displaystyle \cos (\pi - \theta) = -\cos (\theta)$

Therefore, $\displaystyle \cos(\frac{2\pi}{3}) = \cos (\pi - \frac{\pi}{3}) = -\cos (\frac{\pi}{3}) = -\frac{1}{2}$ - Apr 6th 2009, 12:29 PMjepalsolved
ignore this question....i got the answer. :D