Consider a metal rod ($0 < x < l$), insulated along its sides but not at its ends, which is initially at temperature=1. Suddenly both ends are plunged into a bath of temperature=0. Write the PDE, boundary conditions, and initial condition that model the temperature of the rod. Write the formula for the temperature $u(x,t)$ at position $x$ and time $t$ for later times. In this problem assume identity
$$1=(4/\pi)[sin(\pi x/l)+(1/3)sin(3\pi x/l)+(1/5)sin(5\pi x/l)+...]$$
holds for all $0 < x < l$
This appears to be an example of a heat equation.
Let $u(x,t)$ denote the temperature at $x$ at a given time $t$ of a rod of length $l$.
We may thus describe the heat distribution as $$\frac{\partial u}{\partial t} = a^2\frac{\partial^2 u}{\partial x^2}$$ where $a^2$ is known as the thermal diffusivity, which depends on the material which the bar is made of. You can read more about this here.
The above PDE must be considered subject to boundary conditions \begin{align}u(0,t) &= 0 ~~~~~~~ t>0 \\ u(l,t) &= 0 ~~~~~~~ t>0\end{align} with initial condition $$u(x,0) = 1 ~~~~ 0 \le x\le l$$
You may now solve this PDE in order to obtain the solution of $u(x,t)$ such that your given boundary conditions are met. I would suggest Seperation of Variables.