Biologists are studying the evolution of an insect population in a botanical garden. At the beginning of the study, the population is 100,000 insects. To preserve the balance of the natural environment, the number of insects must not exceed 400,000.
Part A: Study of a first model in the laboratory
Observation of the evolution of these insect populations in the laboratory, in the absence of any predator, shows that the number of insects increases by $60\%$ each month. Taking this observation into account, biologists model the evolution of the insect population using a sequence $(u_n)$ where, for every natural number $n$, $u_n$ models the number of insects, expressed in millions, after $n$ months. We therefore have $u_0 = 0.1$.
- Justify that for every natural number $n$: $u_n = 0.1 \times 1.6^n$.
- Determine the limit of the sequence $(u_n)$.
- By solving an inequality, determine the smallest natural number $n$ from which $u_n > 0.4$.
- According to this model, would the balance of the natural environment be preserved? Justify your answer.
Part B: Study of a second model
Taking into account the constraints of the natural environment in which the insects evolve, biologists choose a new model. They model the number of insects using the sequence $(v_n)$, defined by: $$v_0 = 0.1 \text{ and, for every natural number } n, v_{n+1} = 1.6v_n - 1.6v_n^2,$$ where, for every natural number $n$, $v_n$ is the number of insects, expressed in millions, after $n$ months.
- Determine the number of insects after one month.
- We consider the function $f$ defined on the interval $\left[0; \frac{1}{2}\right]$ by $$f(x) = 1.6x - 1.6x^2.$$ a. Solve the equation $f(x) = x$. b. Show that the function $f$ is increasing on the interval $\left[0; \frac{1}{2}\right]$.
- a. Show by induction that, for every natural number $n$, $0 \leqslant v_n \leqslant v_{n+1} \leqslant \frac{1}{2}$. b. Show that the sequence $(v_n)$ is convergent. We denote by $\ell$ the value of its limit. We admit that $\ell$ is a solution of the equation $f(x) = x$. c. Determine the value of $\ell$. According to this model, will the balance of the natural environment be preserved? Justify your answer.
- The threshold function is given below, written in Python language. a. What do we observe if we enter \texttt{seuil(0.4)}? b. Determine the value returned by entering \texttt{seuil(0.35)}. Interpret this value in the context of the exercise. \begin{verbatim} def seuil(a) : v=0.1 n=0 while v