Newton's law of cooling states that the rate of change of the temperature of a body is proportional to the difference between the temperature of this body and that of the surrounding environment.
A cup of coffee is served at an initial temperature of $80^{\circ}\mathrm{C}$ in an environment whose temperature, expressed in degrees Celsius, assumed to be constant, is denoted $M$.
In this part, for any natural number $n$, we denote $T_n$ the temperature of the coffee at instant $n$, with $T_n$ expressed in degrees Celsius and $n$ in minutes. Thus $T_0 = 80$.
We model Newton's law between any two consecutive minutes $n$ and $n+1$ by the equality: $$T_{n+1} - T_n = k(T_n - M)$$ where $k$ is a real constant.
In the rest of part A, we choose $M = 10$ and $k = -0{,}2$. Thus, for any natural number $n$, we have: $T_{n+1} - T_n = -0{,}2(T_n - 10)$.
- Based on the context, can we conjecture the direction of variation of the sequence $(T_n)$?
- Show that for any natural number $n$: $T_{n+1} = 0{,}8T_n + 2$.
- We set, for any natural number $n$: $u_n = T_n - 10$. a. Show that $(u_n)$ is a geometric sequence. Specify its common ratio and its first term $u_0$. b. Show that, for any natural number $n$, we have: $T_n = 70 \times 0{,}8^n + 10$. c. Determine the limit of the sequence $(T_n)$.
- Consider the following algorithm: \begin{verbatim} While $T \geqslant 40$ $T \leftarrow 0,8T + 2$ $n \leftarrow n + 1$ End While \end{verbatim} a. Initially, we assign the value 80 to the variable $T$ and the value 0 to the variable $n$. What numerical value does the variable $n$ contain at the end of the algorithm's execution? b. Interpret this value in the context of the exercise.