In a factory, a kiln bakes ceramics at a temperature of $1000^{\circ}\mathrm{C}$. At the end of baking, it is turned off and cools down. The temperature of the kiln is expressed in degrees Celsius (${}^{\circ}\mathrm{C}$). The kiln door can be opened safely for the ceramics as soon as its temperature is below $70^{\circ}\mathrm{C}$.
In this part, we denote $t$ the time (in hours) elapsed since the moment the kiln was turned off. The temperature of the kiln (in degrees Celsius) at time $t$ is given by the function $f$ defined, for every positive real number $t$, by: $$f(t) = a\mathrm{e}^{-\frac{t}{5}} + b,$$ where $a$ and $b$ are two real numbers. We admit that $f$ satisfies the following relation: $f'(t) + \frac{1}{5}f(t) = 4$.
- Determine the values of $a$ and $b$ knowing that initially, the temperature of the kiln is $1000^{\circ}\mathrm{C}$, that is $f(0) = 1000$.
- For the following, we admit that, for every positive real number $t$: $$f(t) = 980\mathrm{e}^{-\frac{t}{5}} + 20.$$ a. Determine the limit of $f$ as $t$ tends to $+\infty$. b. Study the variations of $f$ on $[0; +\infty[$. Deduce its complete table of variations. c. With this model, after how many minutes can the kiln be opened safely for the ceramics?
- The average temperature (in degrees Celsius) of the kiln between two times $t_1$ and $t_2$ is given by: $\frac{1}{t_2 - t_1}\int_{t_1}^{t_2} f(t)\,\mathrm{d}t$. a. Using the graphical representation of $f$, give an estimate of the average temperature $\theta$ of the kiln over the first 15 hours of cooling. Explain your approach. b. Calculate the exact value of this average temperature $\theta$ and give its value rounded to the nearest degree Celsius.
- In this question, we are interested in the temperature drop (in degrees Celsius) of the kiln over the course of one hour, that is between two times $t$ and $(t+1)$. This drop is given by the function $d$ defined, for every positive real number $t$, by: $d(t) = f(t) - f(t+1)$. a. Verify that, for every positive real number $t$: $d(t) = 980\left(1 - \mathrm{e}^{-\frac{1}{5}}\right)\mathrm{e}^{-\frac{t}{5}}$. b. Determine the limit of $d(t)$ as $t$ tends to $+\infty$. What interpretation can be given to this?