# Math Help - Trigonometry

1. ## Trigonometry

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

2. Originally Posted by Air
Why does $\cos x \cos (\pi / 4) -\sin x \sin (\pi/4)$ equal $\frac{1}{\sqrt2}(\cos x - \sin x)$?

$\cos (\pi / 4) = \sin (\pi / 4) = \frac{1}{\sqrt{2}}$
3. what is $\cos \frac{\pi}{4}$ ? and $\sin \frac{\pi}{4}$
Did you notice that this can also be written as $\cos (x +\frac{\pi}{4})$ ?