Outside the high school program / Other sequences 10 exercises (including 4 corrected)

a
AB 12 1. Fibonacci sequences and numbers E.590 The continuum hypothesis assumes that any part of the line can be cut indefinitely and that each part has a non-zero length. This assumption runs counter to the concept of matter as the mass of an atom, but is widely used in quantum theory for motion phenomena. Infinitesimal calculus is the branch of mathematics that stud-ies the infinitely far and the infinitely near. (paradox of Zeno of Elea) ˇThere is no motion, as the mobile must reach the middle of its path before reaching the endı This paradox is supposed to defeat continuity: i.e., a length can be divided indefinitely. Concept opposite of discrete (atomistic theory) E.1872 Consider a point X moving from A to B describing the entire segment. We break down the point’s path as follows : We’ll say that this point arrives at the first stage, when it has traveled half of [ AB ] : we’ll note X 1 this point. It will be at the second stage when it has covered half of the remaining path, i.e. half of [ X 1 B ] : this point will be noted X 2 . The third step will be half [ X 2 B ] : half the distance restant. . . And so on. Consider X 0 as the starting point A . 1 Place the points X 1 , X 2 , X 3 and X 4 on the straight line below. 2 It will now be assumed that distance AB measures 1 me-ter. a Give the distance X 0 X 1 , X 1 X 2 , X 2 X 3 et X 3 X 4 . b By extrapolation, give the measurements of X 4 X 5 , X 5 X 6 , X 6 X 7 and X 7 X 8 . c For n N , make a guess as to the distance X n 1 X n . 3 We propose to study the infinite sum of terms : S = 1 2 + 1 4 + 1 8 + 1 16 + · · · To do this, for n N , consider the sum to n terms : S n = 1 2 + 1 4 + 1 8 + · · · + 1 2 n a Write, without calculating, the sums S 3 , S 4 and S 5 . b Establish the formula : S n +1 = 1 2 + 1 2 · S n c Give a reason why the numbers S n approach the value 1 when the number of steps becomes very large. We’ll say that S n admits a limit when n tends to + . d Noting S the limit value of S n when n tends to + : we’ll note lim n ↦→ + S n = S . By passing to the limit of the formula 2 , we obtain the equality: S = 1 2 + 1 2 · S . Find the value of S . We’ve just highlighted one of the properties of the continuum hypothesis : Tout segment can be cut into an infinite number of segments all of non-zero length. E.1873 Definition: (from Le Petit Robert) dichotomy: Division, binary subdivision (between two elements that are clearly separated and opposed) Assuming the thesis of continuity, any object is infinitely di-visible. We know that 2 [1 ; 2] because : 1 2 < 2 2 < 2 2 Two numbers and their squares are arranged in the same order. 1 < 2 < 2 Implementation of the algorithm: At step 0 (the initial step) , we know that 2 is contained in the interval [1 ; 2] To move on to the next step, we consider the middle of this interval 1 ; 5 . Then, we compare 1 ; 5 and 2 to deter-mine whether 2 is contained in the interval [1 ; 1 ; 5] or the interval [1 ; 5 ; 2] . This gives us a new interval corresponding to step 1. We repeat this process an infinite number of times. Let [ a n ; b n ] be the interval corresponding to step n and u n its length. 1 a Give the value of u 0 , u 1 , and u 2 . b Place points a 0 , b 0 , a 1 , b 2 , a 3 , and b 4 on the graduated line below. 2 a Give the value of u n based on n . b Give a range for b n a n for n =10 . Deduce the accuracy of the approximation of 2 by u 10 3 What happens to the value of u n when n approaches + ? (We say that n tends toward infinity) . That is, what is the value of lim n ↦→ + u n ? https://chingmath.fr sacados/590 sacados/1872 AB sacados/1873 12
A0T0tt0 A1T1tt1 tt2 E.1874 Achilles and the tortoise (Zeno of Elea’s paradox) Problem : if the tortoise is ahead of Achilles, Achilles will never be able to catch up with it, regardless of his speed ; for when Achilles runs to reach the point from which the tortoise started, the tortoise moves forward in such a way that Achilles can never make up the difference. Let us assume that the tortoise has a one-meter lead, that Achilles runs at a speed of 2 m = s and the tortoise at a speed of 1 m = s . Let t 0 be the starting time, A 0 the initial position of Achilles, and T 0 that of the tortoise. t 1 is the moment when Achilles reaches the turtle’s previous position. Let us denote this position by A 1 . The turtle con-tinues to move forward and finds itself at T 1 . And so on. Based on the fact that Achilles’ and the tortoise’s paths can be divided into as many segments as (continuity hypothesis) , it is believed that Achilles will never catch up with the tor-toise, as there will always be a portion of the path left for it to travel. Another problem : what is the total length of all these small segments when placed end to end? In other words, will Achilles catch up with the tortoise infinitely? 1 Place points A 2 and T 2 on the last graduated line 2 Give the values of T 0 A 0 , T 1 A 1 , T 2 A 2 . 3 We accept the fact that, at any rank in the study, we have : u n = T n A n = 1 2 n . We will calculate how far from the Achilles point the par-ticipants will meet. We note : u n = u 0 + u 1 + u 2 + · · · + u n a Expand : 1 1 2 × S n b Deduce : S n = 1 1 2 n +1 1 1 2 c Give the limit of S n when n tends towards infinity, which we note as : lim n ↦→ + u 0 + u 1 + · · · + u n ou lim n ↦→ + + k =0 u n . Note: Zeno’s paradoxes were reported by Aristotle in his book on physics (Book VI) . We could also have considered the two functions that as- sociate the position of Achilles and the tortoise on the line with time and determined the point of intersection. E.1875 The discontinuity (paradox of Zeno of Elia) E.182 1 Consider the table below : n 0 1 2 3 4 5 6 7 8 9 10 11 F n 0 1 1 2 3 5 8 13 21 34 55 89 Show that, for any natural integer n greater than 2 , the numbers F n verify the following relationship : F n = F n 1 + F n 2 The numbers ( F n ) are called the ˇ numbers of Fibonacci ı ; any sequence of numbers exhibiting such a relationship will be said to be of ˇ Fibonacci suites ı. More precisely, the sequence studied above is defined by the initial conditions : F 0 =0 ; F 1 =1 It is clear that : lim n ↦→ + F n =+ . We will now study the ratio of two consecutive terms ; i.e. we will study the value of the quotient F n +1 F n when n becomes larger and larger (i.e. when n tends towards + ) . 2 a For n =2 , establish the following equality: F n +1 F n = 2 b Complete the following table to within 10 5 : n 1 2 3 4 5 6 7 8 9 10 F n +1 F n 3 Make a conjecture as to the limit of the sequence ( F n ) and the golden number 1+ 5 2 . We will now show that all Fibonacci sequences have the ratio of its consecutive terms tending towards the golden number: 4 Noting R n the value of the ratio F n +1 F n , show that : R n = 1 + 1 R n 1 5 Assuming, that the sequence ( R n ) admits a finite limit r when n tends to + , deduce that r verifies the relation: r 2 = r + 1 6 Verify that 1+ 5 2 and 1 5 2 are two numbers verifying this equation. 7 Assuming the fact that a second-degree polynomial can only cancel in at most two values, say why the sequence of ratios F n +1 F n necessarily tends to 1+ 5 2 2. Jointly defined sequences (final grades) https://chingmath.fr sacados/1874 A0T0tt0 A1T1tt1 tt2 sacados/1875 sacados/182
024681012142468 E.5900 We define the sequences u n and v n on the set N of natural numbers by: u 0 = 0 ; v 0 = 1 ; u n +1 = u n + v n 2 v n +1 = u n + 2 · v n 3 The aim of this exercise is to study the convergence of the sequences u n and v n . Part A 1 Calculate u 1 and v 1 . 2 Consider the function f extracted from an algorithm the parameter n is an integer greater than or equal to 1 : Function f(n) u 0 v 1 For k varying from 1 to n w u u w+v 2 v w+2 · v 3 End For Return ( u ; v) a Call the function f with the value n =2 as its argument. Copy and complete the table below showing the state of the variables during the call to the function f . k w u v 1 2 b For a given integer N , what does the pair of values returned by the function f correspond to in relation to the situation studied in this exercise? Part B 1 a Using recursive reasoning, show that, for any natu-ral number, we have : v n u n > 0 b Deduce that the sequence u n is an increasing se-quence and v n is a decreasing sequence. 2 Justify that the sequences u n and v n are convergent. Part C 1 Consider the sequence w n defined, for any natural num-ber n , by: w n = v n u n a Demonstrate that the sequence w n is geometric. b Justify that the limits of the sequences u n and v n are equal. 2 Consider the sequence t n defined, for any natural num-ber n , by: t n =2 · u n +3 · v n a Demonstrate that the sequence t n is constant. b Deduce the expression of the terms of the sequence u n as a function of n . c Determine the limit of the sequence u n . 3. A little further: homographic sequences E.2454 Consider the sequence u n n N de-fined by: u 0 = 14 ; u n +1 = 10 · u n 1 u n + 8 for any n N . We admit that the sequence u n is defined on N and that the terms of the sequence are strictly greater than 1 . 1 Shown below is the representative curve of the function f whose image of x is defined by the relation: f ( x ) = 10 x 1 x + 8 as well as the representation of the first bisector of the plane : Show the first five terms of the sequence on the x-axis. 2 Consider the sequence v n n N defined by the relation: v n = 1 u n 1 for any natural number n . a Show that for any n N , we have : v n +1 v n = 1 9 b Give the explicit formula defining each term of the se-quence v n as a function of rank n . c Deduce the explicit formula defining each term of the https://chingmath.fr chapExoCorrec/5900 sacados/5900 chapExoCorrec/2454 sacados/2454 024681012142468
-4-3-2-12I-12JOCf sequence u n as a function of rank n . E.2414 Consider the sequence u n n N de-fined : u 0 =4 ; u n +1 = u n +6 u n 2 for any natural number n . 1 Determine the first three terms of the sequence u n . 2 Let v n n N be the sequence defined by the relation: v n = u n + 2 u n 3 for any natural number n . a Determine the first three terms of this sequence. b Show that : v n +1 v n = 1 4 c Deduce the nature of the sequence v n as well as the explicit formula determining the term of rank n as a function of n . 3 a Determine the expression of the term u n as a func-tion of the term v n . b Deduce the explicit formula defining the terms of u n as a function of n . E.5166 Consider the function f defined on −∞ ; 8 by the relation: f ( x ) = 4 · x + 4 x + 8 In the orthonormal reference frame O ; I ; J below, is repre-sented the curve C f representative of the function f . Consider the sequence u n defined by the relations : u 0 = 4 ; u n +1 = f u n for all n N 1 On the graph, plot the first five terms of the suite u n on the x-axis. 2 By calculation, determine the exact value of the first five terms of the sequence u n . 3 Consider the sequence v n defined by the relation: v n = 1 u n 2 for all n N a Determine the first five terms of the sequence v n . b Make a conjecture about the nature of the sequence v n and the value of its characteristic elements. https://chingmath.fr chapExoCorrec/2414 sacados/2414 chapExoCorrec/5166 sacados/5166 -4-3-2-12I-12JOCf