Exercise 2 (5 points)
An online platform offers two types of video games: a game of type $A$ and a game of type $B$.
Part A
The durations of games of type $A$ and type $B$, expressed in minutes, can be modeled respectively by two random variables $X_A$ and $X_B$. The random variable $X_A$ follows the uniform distribution on the interval $[9; 25]$. The random variable $X_B$ follows the normal distribution with mean $\mu$ and standard deviation 3.
- a. Calculate the average duration of a game of type $A$. b. Specify using the graph the average duration of a game of type $B$.
- We choose at random, with equal probability, a game type. What is the probability that the duration of a game is less than 20 minutes? Give the result rounded to the nearest hundredth.
Part B
It is admitted that, as soon as the player completes a game, the platform proposes a new game according to the following model:
- if the player completes a game of type $A$, the platform proposes to play again a game of type $A$ with probability 0.8;
- if the player completes a game of type $B$, the platform proposes to play again a game of type $B$ with probability 0.7.
For a natural number $n$ greater than or equal to 1, we denote $A_n$ and $B_n$ the events: $A_n$: ``the $n$-th game is a game of type $A$.'' $B_n$: ``the $n$-th game is a game of type $B$.'' For any natural number $n$ greater than or equal to 1, we denote $a_n$ the probability of event $A_n$.
- a. Copy and complete the probability tree. b. Show that for any natural number $n \geqslant 1$, we have: $a_{n+1} = 0.5\,a_n + 0.3$.
In the rest of the exercise, we denote $a$ the probability that the player plays game $A$ during his first game, where $a$ is a real number belonging to the interval $[0; 1]$. The sequence $(a_n)$ is therefore defined by: $a_1 = a$, and for any natural number $n \geqslant 1$, $a_{n+1} = 0.5\,a_n + 0.3$.
\setcounter{enumi}{1} - Study of a particular case. In this question, we assume that $a = 0.5$. a. Show by induction that for any natural number $n \geqslant 1$, we have: $0 \leqslant a_n \leqslant 0.6$. b. Show that the sequence $(a_n)$ is increasing. c. Show that the sequence $(a_n)$ is convergent and specify its limit.
- Study of the general case. In this question, the real number $a$ belongs to the interval $[0; 1]$. We consider the sequence $(u_n)$ defined for any natural number $n \geqslant 1$ by $u_n = a_n - 0.6$. a. Show that the sequence $(u_n)$ is a geometric sequence. b. Deduce that for any natural number $n \geqslant 1$, we have: $a_n = (a - 0.6) \times 0.5^{n-1} + 0.6$. c. Determine the limit of the sequence $(a_n)$. Does this limit depend on the value of $a$? d. The platform broadcasts an advertisement inserted at the beginning of games of type $A$ and another inserted at the beginning of games of type $B$. Which advertisement should be the most viewed by a player intensively playing video games?