# a the relation arb is defined if and only if = 2 k, k

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1 DISCRETE MATHEMATICS Past Paper Questions in Number Theory 1. Prove that 3k + 2 and 5k + 3, k are relatively prime. (Total 6 marks) 2. (a) Given that the integers m and n are such that 3 (m 2 + n 2 ), prove that 3 m and 3 n. Hence show that 2 is irrational. 3. (a) Write as a product of primes. Numbers of the form F n = n , n are called Fermat numbers. Find the smallest value of n for which the corresponding Fermat number has more than a million digits. (c) Prove that (a) Explaining your method fully, determine whether or not 1189 is a prime number. (i) State the fundamental theorem of arithmetic. (ii) The positive integers M and N have greatest common divisor G and least common multiple L. Show that GL = MN. (Total 10 marks) 5. (a) Prove that the number is the fourth power of an integer in any base greater than 6. For a, b a the relation arb is defined if and only if = 2 k, k b. (i) Prove that R is an equivalence relation. (ii) List the equivalence classes of R on the set {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}. (8) IB Questionbank Mathematics Higher Level 3rd edition 1

2 6. The positive integer N is expressed in base p as (a n a n 1... a 1 a 0 ) p. (a) Show that when p = 2, N is even if and only if its least significant digit, a 0, is 0. Show that when p = 3, N is even if and only if the sum of its digits is even. 7. Given that n 2 + 2n + 3 N (mod 8), where n and 0 N 7, prove that N can take one of only three possible values. 8. (a) Given that a, b and c +, show that if a 1 (mod c), then ab b (mod c). Using mathematical induction, show that 9 n l (mod 4), for n. (c) The positive integer M is expressed in base 9. Show that M is divisible by 4 if the sum of its digits is divisible by Find the remainder when is divided by 65. (Total 6 marks) 10. Given that a, b, c, d, show that (a b)(a c)(a d)(b c)(b d)(c d) 0 (mod 3). (Total 7 marks) 11. Let a and b be two positive integers. (a) Show that gcd(a,b) lcm(a,b) = ab Show that gcd(a, a + b) = gcd(a, b) (Total 13 marks) 12. (a) Write the number in base 8. Prove that if a number is divisible by 7 that the sum of its base 8 digits is also divisible by 7. (c) Using the result of part, show that the number is not divisible by 7. IB Questionbank Mathematics Higher Level 3rd edition 2

3 13. The positive integer N is expressed in base 9 as (a n a n 1... a 0 ) 9. (a) Show that N is divisible by 3 if the least significant digit, a 0, is divisible by 3. (c) Show that N is divisible by 2 if the sum of its digits is even. Without using a conversion to base 10, determine whether or not ( ) 9 is divisible by (a) Define what is meant by the statement x y (mod n) where x, y, n +. (1) Hence prove that if x y (mod n) then x 2 y 2 (mod n). (c) Determine whether or not x 2 y 2 (mod n) implies that x y (mod n). 15. (a) Show that a positive integer, written in base 10, is divisible by 9 if the sum of its digits is divisible by 9. The representation of the positive integer N in base p is denoted by (N) p. (126) 7 If ( 5 ) 7 = ( anan 1... a1a0 ) 7, find a 0. (Total 16 marks) 16. (a) Using Fermat s little theorem, show that, in base 10, the last digit of n is always equal to the last digit of n 5. Show that this result is also true in base 30. IB Questionbank Mathematics Higher Level 3rd edition 3

4 17. (a) Write down Fermat s little theorem. In base 5 the representation of a natural number X is (k00013(5 k)) 5. This means that X = k (5 k). In base 7 the representation of X is (a n a n 1...a 2 a 1 a 0 ) 7. Find a 0. (c) Given that k = 2, find X in base Prove that if gcd(a, b) = 1 and gcd(a, c) = 1, then gcd (a, bc) = 1. (Total 5 marks) 19. (a) Use the Euclidean Algorithm to find the gcd of 324 and 129. Hence show that 324x + 129y = 12 has a solution and find both a particular solution and the general solution. (c) Show that there are no integers x and y such that 82x + 140y = (a) Use the Euclidean algorithm to find the greatest common divisor of the numbers 56 and 315. (i) Find the general solution to the diophantine equation 56x + 315y = 21. (ii) Hence or otherwise find the smallest positive solution to the congruence 315x 21 (modulo 56). (Total 13 marks) 21. Use the Euclidean Algorithm to find the greatest common divisor of 7854 and Hence state the number of solutions to the diophantine equation 7854x y = 41 and justify your answer. (Total 7 marks) 22. (a) Use the Euclidean algorithm to find gcd (12306, 2976). Hence give the general solution to the diophantine equation 12306x y = 996. IB Questionbank Mathematics Higher Level 3rd edition 4

5 23. (a) Given the linear congruence ax b(mod p), where a, b, p is a prime and gcd (a, p) = 1, show that x a p 2 b (mod p). (i) Solve 17x 14 (mod 21). (ii) Use the solution found in part (i) to find the general solution to the Diophantine equation 17x + 21y = 14. (10) 24. (a) (i) Given that a d (mod n) and b c (mod n) prove that (a + b) (c + d) (mod n). (ii) Hence solve the system 2x+ 5y 1(mod 6) x+ y 5(mod 6) Show that x 97 x (mod 97) has no solution. (11) 25. An arithmetic sequence has first term 2 and common difference 4. Another arithmetic sequence has first term 7 and common difference 5. Find the set of all numbers that are members of both sequences. 26. (a) Convert the decimal number to base 16. (i) Using the Euclidean algorithm, find the greatest common divisor, d, of 901 and 612. (c) (ii) Find integers p and q such that 901p + 612q = d. (iii) Find the least possible positive integers s and t such that 901s 612t = 85. In each of the following cases find the solutions, if any, of the given linear congruence. (i) (ii) 9x 3 (mod18); 9x 3 (mod15). (10) (Total 19 marks) 27. Solve the system of linear congruences x 1(mod 3); x 2(mod 5); x 3(mod 7). (Total 6 marks) IB Questionbank Mathematics Higher Level 3rd edition 5

6 28. (a) Find the general solution for the following system of congruences. N 3 (mod 11) N 4 (mod 9) N 0 (mod 7) Find all values of N such that 2000 N (a) (i) One version of Fermat s little theorem states that, under certain conditions, a p 1 1(mod p). Show that this result is not valid when a = 4, p = 9 and state which condition is not satisfied. (ii) Given that 5 64 n(mod 7), where 0 n 6, find the value of n. (8) Find the general solution to the simultaneous congruences x 3(mod 4) 3x 2(mod 5). 30. (a) Given that ax b (mod p) where a, b, p, x +, p is prime and a is not a multiple of p, use Fermat s little theorem to show that x a p 2 b (mod p). Hence solve the simultaneous linear congruences 3x 4(mod 5) 5x 6(mod 7) giving your answer in the form x c (mod d). (8) 31. Two mathematicians are planning their wedding celebration and are trying to arrange the seating plan for the guests. The only restriction is that all tables must seat the same number of guests and each table must have more than one guest. There are fewer than 350 guests, but they have forgotten the exact number. However they remember that when they try to seat them with two at each table there is one guest left over. The same happens with tables of 3, 4, 5 and 6 guests. When there were 7 guests per table there were none left over. Find the number of guests. (Total 10 marks) IB Questionbank Mathematics Higher Level 3rd edition 6

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