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Mathematical

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Approach Many of these problems read as brain teasers at first, but can be worked through in a logical way. Just remember to rely on the rules of mathematics to develop an approach, and then to carefully translate that idea into code.

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Example Given two numbers m and n, write a method to return the first number r that is divisible by both (e.g., the least common multiple).

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hints What does it mean for r to be divisible by m and n? It means that all the primes in m must go into r, and all primes in n must be in r. What if m and n have primes in common? For example, if m is divisible by 3^5 and n is divisible by 3^7, what does this mean about r? It means r must be divisible by 3^7. The Rule For each prime p such that p^a \ m (e.g., m is divisible by p^a) and p^b \ n, r must be divisible by p^max(a, b).

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Find the LCM of these sets of numbers. 3, 9, 21 Solution: List the prime factors of each. 3: 3 9: 3 × 3 21: 3 × 7 63 can be divided evenly by 3, 9, and 21. 12, 80 Solution: List the prime factors of each. 12: 2 × 2 × 3 80: 2 × 2 × 2 × 2 × 5 = 80 240 can be divided by both 12 and 80.

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Algorithm

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Prime A number is prime if it is only divisible by 1 and itself. So for example 2, 3, 5, 79, 311 and 1931 are all prime, while 21 is not prime because it is divisible by 3 and 7. To find if a number n is prime we could simply check if it divides any numbers below it. We can use the modulus (%) operator to check for divisibility:

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Solution for (int i=2; i

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Implementation public boolean isPrime (int n) { if (n<=1) return false; if (n==2) return true; if (n%2==0) return false; int m=Math.sqrt(n); for (int i=3; i<=m; i+=2) if (n%i==0) return false; return true; }

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Problem Design an algorithm to find the kth number such that the only prime factors are 3, 5, and 7.

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Hints

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3 * (previous number in list) 5 * (previous number in list) 7 * (previous number in list) How would we find the next number in the list? Well, we could multiply 3, 5 and 7 times each number in the list and find the smallest element that has not yet been added to our list. This solution is O(n^2). Not bad, but I think we can do better

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Hints 357 33*33*53*7 55*35*55*7 77*37*57*7 Red: duplications 3*33*53*75*57*7 33*3*33*3*53*3*73*5*53*7*7 55*3*35*5*35*3*75*5*55*7*7 77*3*37*3*57*3*77*5*57*7*7

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Hints In our current algorithm, we’re doing 3*1, 3*3, 3*5, 3*7, 3*9, 3*15, 3*21, 3*25 …, and the same for 5 and 7.We’ve already done almost all this work before—why are we doing it again? We can fix this by multiplying each number we add to our list by 3, 5, 7 and putting the results in one of the three first-in-first-out queues. To look for the next “magic” number, we pick the smallest element in the three queues.

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Solution

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5 Minute Check Determine the missing digit to make the statement true. Complete on the back of your homework. 1. 6, ?59 is divisible by 3. 2.

5 Minute Check Determine the missing digit to make the statement true. Complete on the back of your homework. 1. 6, ?59 is divisible by 3. 2.

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