Chain rule
The chain rule in calculus is a fundamental formula used when performing differentiation of compound functions in one or multiple variables. For real-valued functions of several variables, the chain rule extends to an equation involving partial derivatives.
Chain rule in one independent variable
Suppose \(z\) is a differentiable function in \(x\) and \(y\), and both \(x\) and \(y\) are differentiable functions in \(t\). Then \(z\) will also be a differentiable function in \(t\), and
\[ \frac{dz}{dt} = \frac{\partial z}{\partial x} \frac{dx}{dt} + \frac{\partial z}{\partial y} \frac{dy}{dt}. \]
This is the multivariate version of the familiar chain rule.
Here, the differentiability assumptions are important. When this assumption is violated, the partial derivatives and derivatives can interact in very complicated ways.
Distance to a point
Suppose the trajectory of a moving particle is given by the parametric equations \[ \left\{ \begin{aligned} x &= \cos{\left(t \right)} \\ y &= \sin{\left(t \right)} \\ \end{aligned} \right. \] Let \(z\) be the distance between this particle and the point \((3,4)\). Is \(z\) a differentiable function in \(t\)? If yes, compute the derivative \(\frac{dz}{dt}\).
Distance between two moving points
Suppose the trajectories of two moving particles are given by the parametric equations \[ \begin{aligned} & \left\{ \begin{aligned} x_1 &= \cos{\left(3 t \right)} \\ y_1 &= \sin{\left(3 t \right)} \\ \end{aligned} \right. &&\text{and} & \left\{ \begin{aligned} x_2 &= 2 \cos{\left(t \right)} \\ y_2 &= 2 \sin{\left(t \right)} \\ \end{aligned} \right. \end{aligned} \]
Express the instantaneous rate of change of the distance between the two moving particles as a function of \(t\).
Resistors in a parallel circuit
The total resistance \(R\) (in ohms) of two resistors connected in parallel is given by
\[ \frac{1}{R} = \frac{1}{R_1} + \frac{1}{R_2} \tag{1}\]
Suppose the resistance of both resistors are changing and their values are given by the functions
\[ \begin{aligned} R_1 &= 2 t + 10 & R_2 &= t + 15 \end{aligned} \]
where \(t \ge 0\) represent time (measured in seconds). Is \(R\) a differentiable function of \(t\)? If yes, compute the derivative \(\frac{dR}{dt}\).