# Problems for children from 5 to 15. V. I. Arnold

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1 Problems for children from 5 to 5 V. I. Arnold

2 Abstract This brochure consists of 77 problems for development of thinking culture, either selected or composed by the author. Most of them do not require any special knowledge beyond the general education. However, solving some of them may turn out challenging even for professors. The book is addressed to school and university students, teachers, parents to everybody who considers the thinking culture an essential part of the personality development.

3 I put these problems onto paper in Paris in spring 2004 when Russian Parisians asked me to help their young children gain the thinking culture traditional for Russia. I am deeply convinced that this culture is most of all cultivated by early independent reflection on simple, but not easy questions similar to those below (problems, 3, 3 are the most recommended). My long experience has shown that, very frequently, dimwits falling at school behind solve them better than A-grade pupils, since for their survival at the back of the classroom they must permanently think more than required for governing the whole Seville and Granada, as Figaro used to say about himself, while A-graders cannot catch what should be multiplied by what in these problems. I have also noticed that five year old kids solve similar problems better than pupils spoiled by coaching, which in their turn cope with the questions better than university students used to swotting who anyway beat their professors (the worst in solving these simple problems are Nobel and Fields prize winners). * * *. Masha was seven kopecks short to buy a first reading book, and Misha lacked one kopeck. They combined their money to buy one book to share, but even then they did not have enough. How much did the book cost? 2. A bottle with a cork costs 0 kopecks, while the bottle itself is 9 kopecks more expensive than the cork. How much does the bottle without the cork cost? 3. A brick weighs one pound and half the brick. How many pounds does the brick weigh? 4. A spoon of wine is poured from a barrel of wine into a (not full) glass of tea. After that, the same spoon of the (inhomogeneous) mixture from the glass is taken back into the barrel. Now both in the barrel and in the glass there is a certain volume of the foreign liquid (wine in the glass and tea in the barrel). In which is the volume of the foreign liquid greater: in the glass or in the barrel? 5. Two old ladies left from A to B and from B to A at dawn heading towards one another (along the same road). They met at noon, but did not stop, and each of them carried on walking with the same speed. The first lady 2

4 came (to B) at 4pm, and the second (to A) at 9pm. What time was the dawn that day? 6. The hypotenuse of a right-angled triangle (in a standard American examination) is 0 inches, the altitude dropped onto it is 6 inches. Find the area of the triangle. American school students had been coping successfully with this problem over a decade. But then Russian school students arrived from Moscow, and none of them was able to solve it as had their American peers (giving 30 square inches as the answer). Why? 7. Vasya has 2 sisters more than he has brothers. How many daughters more than sons do Vasya s parents have? 8. There is a round lake in South America. Every year, on June, a Victoria Regia flower appears at its centre (its stem rises from the bottom, and its petals lie on the water like those of a water lily). Every day the area of the flower doubles, and on July, it finally covers the entire lake, drops its petals, and its seeds sink to the bottom. On which date is the flower s area half the area of the lake? 9. A peasant must take a wolf, a goat and a cabbage across a river in a boat. However the boat is so small that he is able to take only one of the three on board with him. How should he transport all three across the river? (The wolf cannot be left alone with the goat, and the goat cannot be left alone with the cabbage.) 0. During the daytime a snail climbs 3 cm up a post, and during the night, falling asleep, accidentally goes down by 2 cm. The post is 0 m high, and a delicious (for the snail) sweet is on its top. In how many days will the snail get the sweet?. A ranger walked from his tent 0 km southwards, turned east, walked straight eastwards 0 km more, met his bear friend, turned north and after another 0 km found himself by his tent. What colour was the bear and where did all this happen? 2. A tide was in today at 2 noon. What time will it be in (at the same place) tomorrow? 3

5 3. Two volumes of Pushkin, the first and the second, are side-by-side on a bookshelf. The pages of each volume are 2 cm thick, and the cover front and back each is 2 mm. A bookworm has gnawed through (perpendicular to the pages) from the first page of volume to the last page of volume 2. How long is the bookworm s track? [This topological problem with an incredible answer 4 mm is absolutely impossible for academicians, but some preschoolers handle it with ease.] 4. Find a body with views from the top and from the front as depicted (polytopes). Depict its side view (showing invisible edges of the polytope dashed). Top view Front view 5. How many ways are there to break up the number 64 into 0 natural summands (integers ), whose maximum is 2? [Ways that differ only by the order of their summands do not count as different.] 6. By putting a few similar bars one onto another (for example, domino pieces), one can make a length x overhang. What is the maximal attainable value of the overhang length x? x 7. The distance between cities A and B is 40 km. Two cyclists leave respectively from A and B simultaneously towards one another, one with speed 0 km/h and the other with speed 5 km/h. A fly flies out with the first cyclist from A with the speed of 00km/h, reaches the second, touches his forehead and flies back to the first, touches his forehead, returnes to the second, and so on until the cyclists foreheads collide and squash the fly. How many kilometers altogether has the fly flown? A B 4

6 8. One domino piece covers two squares of a chessboard. Cover all the squares except for two opposite ones (on the same diagonal) with 3 pieces. [A chessboard consists of 8 8 = 64 squares.] 9. A caterpillar wants to slither from a corner of a cubic room (the left on the floor) to the opposite one (the right on the ceiling). Find the shortest route for such a journey along the walls of the room. 20. You have two vessels of volumes 5 litres and 3 litres. Measure out one litre (obtaining it in one of the vessels) There are five heads and fourteen legs in a family. How many people and how many dogs are in the family? 22. Equilateral triangles are constructed externally on sides AB, BC and CA of a triangle ABC. Prove that their centres ( ) form an equilateral triangle. A B C 5

7 23. What polygons may be obtained as sections of a cube by a plane? Can we get a pentagon? A heptagon? A regular hexagon? 24. Draw a straight line through the centre of a cube so that the sum of squares of the distances to it from the eight vertices of the cube would be a) maximal, b) minimal (comparing with other such lines). 25. A right circular cone is cut by a plane along a closed curve. Two balls insribed into the cone are tangent to the plane, one at point A and the other at point B. Find a point C on the cut line so that the sum of the distances CA + CB would be a) maximal, b) minimal. B A 26. The Earth s surface is projected onto the cyliner formed by the lines tangent to the meridians at their equatorial points along the rays parallel to the equator and passing through the Earth s pole axis. Will the area of the projection of France be greater or smaller than the area of France itself? 6

8 27. Prove that the remainder of division of the number 2 p by an odd prime p is (examples: 2 2 = 3a+, 2 4 = 5b+, 2 6 = 7c+, 2 0 = 023 = 93). 28. A needle 0 cm long is thrown randomly onto lined paper with the distance between neighbouring lines also 0 cm. This is repeated N (a million) times. How many times (approximately, up to a few per cent error) will the fallen needle intersect a line on the paper? One can perform (as I did at the age of 0) this experiment with N = 00 2 instead of a million throws. [The answer to this problem is suprising: N. π Moreover even for a bent needle of length a 0 cm the number of intersections observed over N throws will be approximately 2aN. The number π π ] Polyhedra with triangular faces are, for example, Platonic solids: tetrahedron (4 faces), octahedron (8 of them), icosahedron (20 and all the faces are the same; it is interesting to draw it, it has 2 vertices and 30 edges). tetrahedron (tetra = 4) octahedron (octo = 8)? icosahedron Is it true that for any such (bounded convex polyhedron with triangular faces) the number of faces is equal to twice the number of vertices minus four? 7

9 Yet another Platonic solid (there are 5 of them altogether): 30. A dodecahedron is a convex polyhedron with twelve (regular) pentagonal faces, twenty vertices and thirty edges (its vertices are the centres of the faces of an icosahedron). Inscribe into a dodecahedron five cubes (the vertices of each cube are vertices of the dodecahedron) whose edges are diagonals of faces of the dodecahedron (a cube has 2 edges, one per face). [This was invented by Kepler for the sake of planets.] 3. Find the intersection of two tetrahedra inscribed into a cube (so that the vertices of each are vertices of the cube, and the edges are diagonals of the faces). What fraction of the cube s volume is contained within the tetrahedra s intersection? 3 bis. Construct the section of a cube by the plane passing through three given points on the edges. [Draw the polygon along which the planar section intersects the faces of the cube.] 32. How many symmetries does a tetrahedron have? How many has a cube? octahedron? icosahedron? dodecahedron? A symmetry is a transformation preserving lengths. How many rotations are among them and how many reflections (in each of the five cases listed)? 8

10 33. How many ways are there to paint 6 faces of similar cubes with six colours (,...,6) [one per face] so that no two of the coloured cubes obtained would be the same (could not be transformed one into another by a rotation)? How many different ways are there to permute n objects? There are six of them for n = 3: (,2,3), (,3,2), (2,,3), (2,3,), (3,,2), (3,2,). What if the number of objectis is n = 4? n = 5? n = 6? n = 0? A cube has 4 long diagonals. How many different permutations of these four objects are obtained by rotations of a cube? 36. The sum of the cubes of three integers is subtracted from the cube of the sum of these numbers. Is the difference always divisible by 3? 37. Same question for the fifth powers and divisibility by 5, and for the seventh powers and divisibility by Calculate the sum (with an error of not more than % of the answer). 9

11 39. If two polygons have equal areas, then they may be cut into a finite number of polygonal parts which may then be rearranged to obtain both the first and second polygons. Prove this!. [For spatial solids this is not the case: the cube and tetrahedron of equal volumes cannot be cut this way!] 40. Four vertices of a parallelogram have been chosen at nodes of a piece of squared paper. It turns out that neither the parallelogram s sides nor its interior contain any other nodes of the squared paper. Prove that the area of such a parallelogram is equal to the area of one square of the paper. a=2, b=2 4. Under the conditions of question 40, a nodes have turned out to be in the interior and b on the sides of the parallelogramm. Find its area. 42. Is the statement analogous to question 40 true for parallelepipeds in 3- space? 43. The rabbit (or Fibonacci) numbers form a sequence (a = ),, 2, 3, 5, 8, 3, 2, 34,..., in which a n+2 = a n+ + a n for any n =, 2,.... Find the greatest common divisor of the numbers a 00 and a 99. 0

12 44. Find the (Catalan) number of ways to cut a convex n-gon into triangles by cutting along its non-intersecting diagonals. For example, c(4) = 2, c(5) = 5, c(6) = 4. How can one find c(0)? 45. A cup tournament has n participating teams, each losing team leaves, and the overall winner is decided after n games. The tournament schedule may be written symbolically as, for instance, ((a, (b, c)), d) meaning b plays c, the winner meets a, and the winner of those meets d]. What is the number of different schedules for 0 teams? For 2 teams, we have only (a, b), and the number is. For 3 teams, there are only ((a, b), c), or ((a, c), b), or ((b, c), a), and the number is 3. For 4 teams: (((a, b), c), d) (((a, c), b), d) (((a, d), b), c) (((b, c), a), d) (((b, d), a), c) (((c, d), a), b) (((a, b), d), c) (((a, c), d), b) (((a, d), c), b) (((b, c), d), a) (((b, d), c), a) (((c, d), b), a) ((a, b), (c, d)) ((a, c), (b, d)) ((a, d), (b, c)) 46. Join n points, 2,..., n by intervals (n of them) to obtain a tree. How many different trees may be obtained (the n = 5 case is already interesting!)? n = 2: n = 3: n = 4: 2, the number is ; 2 3, 2 3, 3 2, the number is 3; 2 3 4, 2 3 4, 3 2 4, 4 2 3, , the number is 6.

13 47. A permutation (x, x 2,..., x n ) of numbers {, 2,..., n} is called a snake (of length n) if x < x 2 > x 3 < x Example. n = 2, only < 2, the number is, } n = 3, < 3 > 2, the number is 2, 2 < 3 > n = 4, < 3 > 2 < 4 < 4 > 2 < 3 2 < 3 > < 4, the number is 5. 2 < 4 > < 3 3 < 4 > < 2 Find the number of snakes of length Let s n be the number of snakes of length n: s =, s 2 =, s 3 = 2, s 4 = 5, s 5 = 6, s 6 = 6. Prove that the Taylor series of the tangent is tan x = x! + 2 x3 x ! 5! + = s 2k k= x 2k (2k )!. 49. Find the sum of the series + x2 2! + 5 x4 x ! 6! + = s 2k k=0 x 2k (2k)!. 50. For s >, prove the identity p=2 p s = (the product is over all prime numbers p, and the summation over all natural numbers n). n= n s 2

14 5. Find the sum of the series = n= (prove that it is π 2 /6, that is, approximately 3/2). 52. Find the probability of the irreducibility of a fraction p/q (this is defined as follows: in the disk p 2 + q 2 R 2, we count the number N of vectors with integer p and q not having a common divisor greater than, after which the probability of the irreducibility is the limit of the ratio N(R)/M(R), where M(R) is the number of integer points in the disk (M πr 2 )). n 2 M(5) = 8, N(5) = 44, N/M = 44/8 53. For the sequence of Fibonacci numbers a n from problem 43, find the limit of the ratio a n+ /a n when n tends to infinity: a n+ a n = 2, 3 2, 5 3, 8 5, 3 8, Answer: the golden ratio, 5+,68. [This is the ratio of the sides 2 of a card which stays similar to itself after cutting off the square whose side is the smaller side of the card, AB = P C.] How is the golden ratio related to a BC CD regular pentagon and a five-pointed star? B P C A Q D 3

15 54. Calculate the infinite continued fraction = a 0 + a + a 2 + a [ ] a2k = a 2k+ = 2 (that is, find the limit of the fractions a 0 + a + a for n ). 55. Find the polynomials y = cos 3(arccos x), y = cos 4(arccos x), y = cos n(arccos x), where x. 56. Calculate the sum of the k-th powers of the n complex n-th roots of unity. 57. On the (x, y)-plane, draw the curves defined parametrically by: {x = cos 2t, y = sin 3t}, {x = t 3 3t, y = t 4 2t 2 }. a n 58. Calculate (with an error of not more than 0% of the answer) 2π 0 sin 00 x dx. 59. Calculate (with an error of not more than 0% of the answer) 0 x x dx. 4

16 60. Find the area of a triangle with angles (α, β, γ) on a radius sphere, whose sides are great circles (sections of a sphere by planes passing through its centre). Answer: S = α + β + γ π (for example, for a triangle with three right angles, S = π/2, that is, one-eighth of the total area of the sphere). γ α β 6. A circle of radius r rolls (without slipping) inside a circle of radius. Draw the whole trajectory of a point of the rolling circle (this trajectory is called a hypocycloid) for r = /3, for r = /4, for r = /n, and for r = / In a class of n pupils, estimate the probability of there being two pupils with the same birthdays. Is it high or is it low? Answer: (very) high if the number of the pupils is (well) above n 0, (very) low if it is (well) below n 0, and what this n 0 actually is (when the probability p /2) is to be found. 63. Snell s (Snellius ) law states that the angle α made by a ray of light with the normal to layers of a stratified medium satisfies the equation n(y) sin α = const, where n(y) is the refractive index of the layer at height y (the quantity n is inversely proportional to the speed of light in the medium when taking its speed in vacuum as ; in water n = 4/3). y α y n(y) Draw ray trajectories in the medium air over a desert, where the index n(y) has a maximum at a certain height (a solution to this problem explains mirages in a desert to those understanding how trajectories of rays emanating from objects are related to the images). 5

17 64. Inscribe into an acute-angled triangle ABC a triangle KLM of minimal perimeter (with its vertex K on AB, L on BC, M on CA). K B L A M C Hint: The answer for non-acute-angled triangles is not similar to the beautiful anwer for acute-angled ones. 65. Calculate the mean value of the function /r (where r 2 = x 2 + y 2 + z 2, r is the distance to the origin) on the radius R sphere centred at the point (X, Y, Z). Hint: The problem is related to Newton s gravitation law and Coulomb s law of electricity theory. In the two-dimensional version of the problem, the function should be replaced by ln r, and the sphere by the cirle. 66. The fact 2 0 = implies log Estimate how much they differ, and calculate log 0 2 to three decimal places. 67. Find log 0 4, log 0 8, log 0 5, log 0 50, log 0 32, log 0 28, log 0 25, log 0 64 with the same precision. 68. Using , find an approximate value of log Knowing log 0 64 and log 0 7, find log 0 9, log 0 3, log 0 27, log 0 6, log Using ln( + x) x (ln is log e ), find log 0 e and ln 0 from the relation log 0 a = ln a ln 0 and from the values of log 0 a calculated earlier (for example, for a = 28/25, 024/000 and so on). The Euler number e = 2, is defined as the limit of the sequence ( + n) n for n, and is equal to the sum of the series +! + 2! + 3! It may also be defined ln(+x) via the quoted formula for ln( + x): lim x 0 x =. 6

18 [Solutions to problems deliver after half an hour a table of four-digit logarithms of any numbers using products of numbers found already as basic data and the formula ln( + x) x x2 2 + x3 3 x for corrections.] In this way Newton compiled a table of 40-digit logarithms!. 7. Consider the sequence of powers of two:, 2, 4, 8, 6, 32, 64, 28, 256, 52, 024, 2048,.... Among the first twelve numbers, four have their decimal expression starting with, and none has it starting with 7. Prove that in the limit n the first digit of the numbers 2 m, 0 m n, will be met with a certain frequency: p 30%, p 2 8%,..., p 9 4%. 72. Verify the behaviour of the first digits of powers of three:, 3, 9, 2, 8, 2, 7,.... Prove that, in the limit here, we also get certain frequencies and, moreover, the same as for the powers of two. Find an exact formula for p,..., p 9. Hint: The first digit of a number x is determined by the fractional part of the number log 0 x, therefore one has to consider the sequence of the fractional parts of the numbers mα, where α = log 0 2. Prove that these fractional parts are distributed over the interval from 0 to uniformly: out of the n fractional parts of the numbers mα, 0 m < n, a subinterval A will contain the quantity k n (A) such that, for n, lim(k n (A)/n) = (the length of the subinterval A). 73. Let g : M M be a smooth map of a bounded domain M onto itself which is one-to-one and preserves areas (volumes in the multi-dimensional case) of domains. Prove that in any neighbourhood U of any point of M and for any N there exists a point x such that g T x is also in U for a certain integer T > N ( the recurrence theorem ). 74. Let M be the torus surface (with coordinates α (mod 2π), β (mod 2π)), and g(α, β) = (α +, β + 2) (mod 2π). Prove that the sequence of points {g T (x)}, T =, 2,..., is everywhere dense in the torus. 75. In the notation of problem 74, let g(α, β) = (2α + β, α + β) (mod 2π). Prove that there is an everywhere dense subset of the torus consisting of periodic points x (that is, such that g T (x) x = x for certain integer T > 0). 7

19 76. In the notation of problem 74 prove that, for almost all points x of the torus, the sequence of points {g T (x)}, T =, 2,..., is everywhere dense in the torus (points x without this property constitute a set of measure zero). 77. In problems 74 and 76 prove that the sequence {g T (x)}, T =, 2,..., is distributed over the torus uniformly: if a domian A contains k n (A) points out of the n with T =, 2,..., n, then k n (A) lim n n = mes A mes M (for example, for a Jordan measurable domain A of measure mes A). Note to problem 3. I tried to illustrate with this problem the difference in approaches to tasks by mathematicians and physicists, in my invited paper in the journal Physics Uspekhi for the 2000 Christmas jubilee. My success surpassed by far what I intended: the editors, unlike preschoolers, on whom I had been basing my experience, failed to solve the problem, and therefore altered it to match my 4mm answer as follows: instead of from the first page of volume to the last page of volume 2 they typeset from the last page of volume to the first page of volume 2. This true story is so implausible that I am including it here: the proof is the editors version published by the journal. * * * Translated by Victor Goryunov and Sabir Gusein-Zade from the original Russian edition: Moscow, MCCME, 2004 ISBN Picture credits title page: Archives of the Mathematisches Forschungsinstitut Oberwolfach. 8

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