Grade 8 / Arithmetic 16 exercises (100% corrected)

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23456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899 r298-1 r114-0 1. Number of divisors E.9073 Definition: Let a and b be two positive integers. We say that b is a divisor of the number a if there exists an integer k such that : a = k × b 1 If possible, complete the dotted lines below using inte-gers : a 18 = 1 × : : : b 18 = 2 × : : : c 18 = 3 × : : : d 18 = 4 × : : : e 18 = 5 × : : : f 18 = 6 × : : : g 18 = 7 × : : : h 18 = 8 × : : : i 18 = 9 × : : : 2 Give the set of divisors of the number 18 . E.9074 a Give the 2 factors of the number 13 . b Give the 6 factors of the number 12 . c Give the 4 factors of the integer 22 . E.9075 1 a Give all divisors of the integer 8 . b Give all divisors of the integer 9 . 2 Give the 12 divisors of the integer 8 × 9 . E.9322 Say whether the following statement is true or false : Assertion: for all integers n between 2 and 9 , 2 n 1 is a prime number. E.9326 Determine the smallest odd positive integer that has three different prime factors. Ex-plain your reasoning. 2. First integers E.9076 The table below shows the integers from 2 to 99 : 1 a Circle the integer 2 . b Hash the boxes whose integer admits 2 as a factor. 2 a Circle the integer 3 . b Hash the boxes whose integer admits 3 as a factor. 3 a Repeat the previous instructions with the integers 5 and 7 . b Circle all of the nonhidden numbers on the grid. 4 a How many factors do the integers 2 , 3 , 5 , 7 , 11 ad-mit? b Which of the integers 13 , 17 , 19 , 23 , 29 , 31 , 37 , 41 , 47 , belong to a multiplication table? If so, which one? c For what reason can we say that the number 97 admits only two factors 1 and 97 ? Definition: an integer a is said to be prime if it admits exactly 2 factors : 1 and itself. Example: the integers 2 , 3 , 5 , 7 , 11 , 13 are prime integers. Note: this exercise introduces the so-called ˇ sieve method of Eratosthène ı for quickly determining the prime integers among 2 to 99 . Eratosthenes was a Greek scientist of the 2 e century BC. He is known for being appointed director of the library of Alexandria and for being the first to measure the circumference of the Earth. A slide show featuring the Eratosthenes sieve can be found in the link to the right. E.9077 1 Which of the following integers is not a prime integer: 31 ; 33 ; 37 ; 41 2 Which of the following is not a prime integer: 43 ; 47 ; 49 ; 53 In the link to the right you will find the list of prime integers less than or equal to 1 000 . E.9078 Each of the statements below is false. For each of these statements, find a counterexample to dis-prove it: 1 The sum of two prime integers is a prime integer. 2 The multiplication of two prime integers is a prime inte-ger. 3 All odd integers are prime integers. 3. Decomposition of an integer into a product of prime numbers https://chingmath.fr chapExoCorrec/9073 sacados/9073 chapExoCorrec/9074 sacados/9074 chapExoCorrec/9075 sacados/9075 chapExoCorrec/9322 sacados/9322 chapExoCorrec/9326 sacados/9326 chapExoCorrec/9076 sacados/9076 23456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899 r298-1 chapExoCorrec/9077 sacados/9077 r114-0 chapExoCorrec/9078 sacados/9078
722×36722×2×18722×2×2×9722×2×2×3×3 E.9079 Definition: an integer is said to be decomposed into a product of prime integers if it is written as a product whose factors are all prime integers. Example: the integer 72 admits the decomposition : 2 × 2 × 2 × 3 × 3 Algorithm : opposite is a method of obtaining the decomposition of an inte-ger into a product of prime integers : 1 Use the equality 28=2 × 14 to obtain the prime product decomposition of the integer 28 . 2 Use equality 40=2 × 20 to obtain the prime product de-composition of the integer 40 . 3 Use equality 96=2 × 48 to obtain the prime product de-composition of the integer 96 . E.9080 For each of the integers 198 , 297 , 462 , determine their decomposition into a product of primes. Hints: for this, use the equalities: 198 = 6 × 33 297 = 9 × 33 462 = 14 × 33 E.9081 1 Justify that the number 102 is divisible by 3 . 2 Decompose 102 into products of prime factors. 3 Give 3 nonprime factors of the number 102 . E.9321 What is the largest prime number that divides 41 895 ? E.9324 1 a Determine the prime factor product decomposition of 2744 . b Determine the prime factor product decomposition of 2744 2 . c Using this decoposition, find x such that : x 3 =2744 2 2 Let a and b be two integers greater than 2 such that : a 3 = b 2 a Calculate b when a =100 . b Determine two integers a and b greater than 2 and less than 10 that verify the equality a 3 = b 2 . 4. Simplification of fractions E.9082 1 Give the product decomposition of the prime integers of the integers : 42 ; 90 2 Derive the reduced writing of the fraction 42 90 . E.9083 1 Decompose 140 and 870 into the product of primes. 2 Deduce the irreducible form of the fraction 140 870 E.9084 1 Decompose, without justification, the numbers 1 386 and 1 716 into products of prime factors. 2 Deduce the irreducible form of the fraction : 1 386 1 716 https://chingmath.fr chapExoCorrec/9079 sacados/9079 722×36722×2×18722×2×2×9722×2×2×3×3 chapExoCorrec/9080 sacados/9080 chapExoCorrec/9081 sacados/9081 chapExoCorrec/9321 sacados/9321 chapExoCorrec/9324 sacados/9324 chapExoCorrec/9082 sacados/9082 chapExoCorrec/9083 sacados/9083 chapExoCorrec/9084 sacados/9084