Derivatives of Tangent, Cotangent, Secant, and Cosecant
We can get the derivatives of the other four trig functions by applying
the quotient rule to sine and cosine. For instance,
\begin{eqnarray*} \frac{d }{dx}\big( \tan(x)\big) & = & \left (\frac{\sin(x)}{\cos(x)}
\right )' \cr
& = & \frac{\cos(x) (\sin(x))' - \sin(x) (\cos(x))'}{\cos^2(x)} \cr
& = & \frac{\cos^2(x) + \sin^2(x)}{\cos^2(x)} \cr
& = & \frac{1}{\cos^2(x)} \cr &=& \sec^2(x).
\end{eqnarray*}