We can understand the derivatives of the sine and cosine functions
both algebraically and graphically. In the first video we take a graphic
look:
Algebraically
To get the derivatives algebraically, we need the two limits we derived earlier:
$$\lim_{x \rightarrow 0} \frac{\sin(x)}{x} =1
\qquad \mbox{and} \qquad \lim_{x \rightarrow 0} \frac{\cos(x) -1}{x} = 0,$$
as well as our addition-of-angle formulas.