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Slideshow 1, Mr Richard Sasaki, Room 307 Rounding Numbers – Part 1

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Objectives Understand the concept of rounding Understand the meaning of decimal places, truncation and rounding up Be able to round numbers up and down

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Introduction Welcome to Grade 9 Maths! Things will be a bit easier for a while. Chapter 1 is always easier than the others. We’ll start by learning about rounding. What’s rounding? Changing a number so it’s easier to work with. Basically, we soften the number. Then its easier to calculate with!

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Introduction What’s an easy sounding number close to 9.972? 10 Solving 10 × 3.6 sounds much easier than 9.972 × 3.6. The answer is very close! 10 × 3.6 =36 9.972 × 3.6 =35.8992 If we don’t have to be perfect, this is fine! There is an 0.1008 inaccuracy. However, we need to understand rounding rules. First, let’s look at decimal places.

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Decimal Places Look at the decimal number below. 24.9763 It has decimal places. 4 1. Let’s round it to 3 decimal places. 24.9763 is between 24.976 and 24.977. Which is closer? 24.976 2. Let’s round it to 2 decimal places. 24.9763 is between 24.97 and 24.98. Which is closer? 24.98 We round up and down to whatever is closest.

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Answers 3 3.28 3.3 3 4.015 4.02 4.0 (keep the zero!) 57.3159 57.32 57 0.7924 0.79 1 43.2948 43.29 43 6.2364 6.24 6 284.3349 284.33 284 7.2949 7.29 7 4.0001 4.00 4 1.9997 2.00 2

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A number was rounded to 2 decimal places as below. 24.55 Does this mean that the number must be 24.6 rounded to 1 decimal place? Tricky Question No…it doesn’t. If the number is 24.5 rounded to 1 decimal place, what could it have been? 24.549 1 d.p 24.5 2 d.p 24.55 1 d.p 24.6 Rounding twice (or more) can be bad! It increases inaccuracy.

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Some units can’t be split up, so we truncate. Truncation What is truncation? Truncation has the same effect as rounding down. We cut off the remainder. Can you think of something in life that we truncate? An example is money. If we divide 89 Yen by 10, we can’t have 8.9 Yen. We remove the 0.9 Yen as it doesn’t make sense. So we are left with 8 Yen.

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Sometimes we need to round up. Rounding Up How many 1 litre bottles do we need to hold 3.1 litres? 4 We can’t compress the water! For the next worksheet, we need to decide whether we truncate or round up. How many 1 litre bottles would 3.1 litres fill? 3

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Answers 3 4 8kg 5 5 4 2 3 Yen 20 178 Yen

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