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#1

$Old$ June 19th, 2008, 08:05 AM

Simplicity $Simplicity is offline$

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Why does $\cos x \cos (\pi / 4) -\sin x \sin (\pi/4)$ equal $\frac{1}{\sqrt2}(\cos x - \sin x)$ ?

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$Old$ June 19th, 2008, 08:08 AM

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Why does $\cos x \cos (\pi / 4) -\sin x \sin (\pi/4)$ equal $\frac{1}{\sqrt2}(\cos x - \sin x)$ ?

Thanks in advance.

$\cos (\pi / 4) = \sin (\pi / 4) = \frac{1}{\sqrt{2}}$

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#3

$Old$ June 19th, 2008, 08:10 AM

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what is $\cos \frac{\pi}{4}$ ? and $\sin \frac{\pi}{4}$

Did you notice that this can also be written as $\cos (x +\frac{\pi}{4})$ ?

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