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Mr Fs Maths Notes Number 2. Prime Factors, HCF & LCM

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1. Prime Factors Any positive integer can be written as a product of its prime factors. Now, that may sound complicated, but all it means is that you can break up any number into a multiplication of prime numbers, and its really easy to do with Factor Trees! Dont Forget: 1 is NOT a prime number, so will NEVER be in your factor tree e.g. Express 60 as a product of its prime factors You can break the number up however you like: 6 x 10 or 12 x Continue breaking up each new number into a multiplication Stop when you reach a Prime Number and put a ring around it Check your answer by multiplying all the numbers together 3225 x xx 3 x 2 x 2 x 5 = x x 3 x 2 x 2 x 5 = x

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Look: Even though we started a different way, we still ended up with the same answer! Now, it looks good if you write your answer starting with the smallest numbers: So: 60 = 2 x 2 x 3 x 5 And if you want to be really posh, you can use indices: So: 60 = 2 2 x 3 x 5 Now well do a harder one, but the technique is just the same. e.g. Express 360 as a product of its prime factors You can break the number up however you like. I just went for 36 x 10 because it was easy to spot Continue breaking up each new number into a multiplication Stop when you reach a Prime Number and put a ring around it Check your answer by multiplying all the numbers together 6625 x xx 3 x 2 x 3 x 2 x 2 x 5 = x x Write the numbers in order If you can, use indices 360 = 2 x 2 x 2 x 3 x 3 x = 2 3 x 3 2 x 5

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2. Highest Common Factor The Highest Common Factor (HCF) of two numbers, is the highest number that divides exactly into both 3. Lowest Common Multiple The Lowest Common Multiple (LCM) of two numbers, is the lowest number that is in the times table of both your numbers Now, you can find both of these by trial and error, but I will show you a better way! e.g. Find the LCM and HCF of 24 and 40 First, use Factor Trees to express your numbers as products of their prime factors: x x 2 3 x x xx 24 = 2 x 2 x 2 x 3 40 = 2 x 2 x 2 x 5

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Now, write your answers on top of each other, like this: 24 = 2 x 2 x 2 x 3 40 = 2 x 2 x 2 x 5 Draw two inter-locking circles, and label one 24 and the other Any numbers that appear in both answers go in the middle (the three 2s). The numbers left over go in the circle they belong to Now, here is the clever bit: To get the Highest Common Factor you just multiply all the numbers in the middle So, HCF = 2 x 2 x 2 = 8 To get the Lowest Common Multiple you just multiply every number you can see So, LCM = 3 x 2 x 2 x 2 x 5 = 120

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Good luck with your revision!

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