# Thread: Odd Trig Identity

1. ## Odd Trig Identity

How do I manipulate $sin(x)+cos(x)$ to show that it equals $\sqrt2cos(\frac{\pi}{4}-x)$

Thanks!

2. Hello, RubyRed!

How do I manipulate $\sin x +\cos x$ to show that it equals $\sqrt{2}\,\cos\left(\frac{\pi}{4}-x\right)$
We have: . $\sin x + \cos x$

Multiply by $\frac{\sqrt{2}}{\sqrt{2}}\!:\quad \frac{\sqrt{2}}{\sqrt{2}}(\sin x + \cos x) \;=\;\sqrt{2}\left(\frac{1}{\sqrt{2}}\sin x + \frac{1}{\sqrt{2}}\cos x\right)\;\;{\color{blue}[1]}$

Note that: . $\sin\frac{\pi}{4} = \frac{1}{\sqrt{2}}\;\;\text{ and }\;\;\cos\frac{\pi}{4} = \frac{1}{\sqrt{2}}$

Substitute into [1]: . $\sqrt{2}\left(\sin\frac{\pi}{4}\sin x + \cos\frac{\pi}{4}\cos x\right)$

. . . . . . . . . . . . . $= \;\sqrt{2}\left(\cos\frac{\pi}{4}\cos x + \sin\frac{\pi}{4}\sin x\right)$

. . . . . . . . . . . . . $= \;\sqrt{2}\,\cos\left(\frac{\pi}{4} - x\right) \;\;{\color{red}**}$

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

** . The last step uses the identity:

. . . . $\cos A\cos B + \sin A\sin B \;=\;\cos(A - B)$