Cécile has invited friends to lunch on her terrace. For dessert, she has planned an assortment of individual cakes that she bought frozen. She takes the cakes out of the freezer at $- 19 ^ { \circ } \mathrm { C }$ and brings them to the terrace where the temperature is $25 ^ { \circ } \mathrm { C }$. After 10 minutes, the temperature of the cakes is $1.3 ^ { \circ } \mathrm { C }$.
I- First model
We assume that the thawing rate is constant, that is, the temperature increase is the same minute after minute. According to this model, determine what the temperature of the cakes would be 25 minutes after they are taken out of the freezer. Does this model seem relevant?
II - Second model
We denote $T _ { n }$ the temperature of the cakes in degrees Celsius, after $n$ minutes following their removal from the freezer; thus $T _ { 0 } = - 19$. We assume that to model the evolution of temperature, we must have the following relation
$$\text { For all natural integers } n , T _ { n + 1 } - T _ { n } = - 0.06 \times \left( T _ { n } - 25 \right) \text {. }$$
- Justify that, for all integers $n$, we have $T _ { n + 1 } = 0.94 T _ { n } + 1.5$
- Calculate $T _ { 1 }$ and $T _ { 2 }$. Give values rounded to the nearest tenth.
- Prove by induction that, for all natural integers $n$, we have $T _ { n } \leqslant 25$.
Returning to the situation studied, was this result foreseeable?
4. Study the direction of variation of the sequence $( T _ { n } )$.
5. Prove that the sequence $( T _ { n } )$ is convergent. 6. We set for all natural integers $n$, $U _ { n } = T _ { n } - 25$. a. Show that the sequence $( U _ { n } )$ is a geometric sequence and specify its common ratio and first term $U _ { 0 }$. b. Deduce that for all natural integers $n$, $T _ { n } = - 44 \times 0.94 ^ { n } + 25$. c. Deduce the limit of the sequence $( T _ { n } )$. Interpret this result in the context of the situation studied. 7. a. The manufacturer recommends consuming the cakes after half an hour at room temperature following their removal from the freezer. What is then the temperature reached by the cakes? Give a value rounded to the nearest integer. b. Cécile is a regular customer of these cakes, which she likes to enjoy while still fresh, at a temperature of $10 ^ { \circ } \mathrm { C }$. Give a range between two consecutive integers of the time in minutes after which Cécile should enjoy her cake. c. The following program, written in Python language, must return after its execution the smallest value of the integer $n$ for which $T _ { n } \geqslant 10$.
\begin{verbatim} def seuil() : n=0 T= while T T= n=n+1 return \end{verbatim}
Copy this program onto your paper and complete the incomplete lines so that the program returns the expected value.