$$\frac{lm}{x\rightarrow0}; \frac{\sin\ \phi x}{\sin\ bx}$$
$\lim_{x\to 0}\frac{\sin(\pi x)}{\sin(bx)}$
$\frac{\mathrm{d}}{\mathrm{d}b}(\lim_{x\to 0}\frac{\sin(\pi x)}{\sin(bx)})$
