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Be the number of all natural numbers that can be

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Unformatted text preview: be the number of all natural numbers that can be written with the digits 1 and 2 only and such that the sum of all digits is equal to N . We can compute A N recursively: • If N = 1, then there is only one such number: the number 1. Thus A 1 = 1. • If N = 2, then there are two such numbers: 11 and 2. Thus A 2 = 2. • To compute A N for an arbitrary N , note that any number made out of 1’s and 2’s only the first digit of the number must be either 1 or 2. If we start with 1, then we can complete the number in A N − 1 ways, while if we start with 2 we can complete the number in A N − 2 ways. Therefore A N = A N − 1 + A N − 2 . Using this formula we compute A 1 = 1 , A 2 = 2 , A 3 = 3 , A 4 = 5 , A 5 = 8 , A 6 = 13 , A 7 = 21 , A 8 = 34 , A 9 = 55 , A 10 = 89 , A 11 = 144 , A 12 = 233 , A 13 = 377 . The correct answer is (f) . square Solution of problem 1.2: Dividing 2012 by 7 we get a quotient 287 and remainder 3. Thus 2012 equals 3 modulo 7, and so 2012 2012 = 3 2012 modulo 7 . Computing the first few powers of 3 modulo 7 we see that 3 2 = 9 = 2 modulo 7 , 3 3 = 27 =- 1 modulo 7 . In particular since 2012 = 3 · 670 + 2 we get 3 2012 = (3 3 ) 670 · 3 2 = (- 1) 670 · 2 = 2 modulo 7 . The correct answer is (a) . square 12 Solution of problem 1.3: (i) Let q = (5 √ 3) / 7. Then the irrational num- ber √ 3 is equal to (7 q ) / 5. If q is rational, since a product or ratio of rational numbers is rational, we will get that √ 3 is rational which is a contradicton. Thus (5 √ 3) / 7 is irrational. (ii) The number q = 2( π- 1) can not be rational. Indeed the irrational number π is equal to 1 + q/ 2. But a ratio and sum of rational numbers is rational so if q is rational we will get that π is rational, which is a contradiction. Thus 2( π- 1) is irrational. (iii) The expression simplifies: 8 √ 8 / 72 = 8 √ 1 / 9 = 8 1 / 3 = 3 √ 8 = 2 . Thus 8 √ 8 / 72 is rational. (iv) The expression simplifies: parenleftBig √ 2 parenrightBig √ 16 = parenleftBig √ 2 parenrightBig 4 = 2 2 = 4 . Thus ( √ 2 ) √ 16 is rational. The correct answer is (c) . square Solution of problem 1.4: (1) is True . There is a one-to-one correspon- dence between the set of all natural numbers and the set of all natu- ral numbers beginning with 3141592. The correspondence is given by matching a number N eginning with 3141592 with the natural number N ′ obtained from N by deleting the leading digits 3141592. (2) is False . We argued in class that for every two real numbers a < b there exists a rational number c such that a < c < b . 13 (3) False . The set of all rational numbers with denominators between 1 and 1000 is infinite: it contains the natural numbers since they can be viewed as rational numbers with denominators 1. The set of all natural numbers between 1 and 1000 is finite. Thus the two sets can not have the same cardinality....
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be the number of all natural numbers that can be written...

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