What is the limit of this trig function? How do I find $$\lim_{x \to \pi/4}{\frac{\cos x-\frac{1}{\sqrt2}}{x-\frac\pi4}}$$? 
I've tried setting the denominator equal to $h$, then replacing $x$ in terms of $h$, but I still don't know how to manipulate it to look like $\frac{\cos h-1}h$.

$$\displaystyle{\lim_{x\to\frac\pi4}\frac{\cos x-\frac{1}{\sqrt2}}{x-\frac\pi4}}$$
$$=\displaystyle{\lim_{x\to\frac\pi4}\frac{\cos x-\cos\frac\pi4}{x-\frac\pi4}}$$
$$=\frac{d(\cos x)}{dx}_{(\text{ at }x=\frac\pi4)}$$
