Transcript Directed Numbers

Form 1 Mathematics
Chapter 5
ESTIMATION
IN NUMBERS AND MEASUREMENT
REMINDER
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Lesson requirement
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Textbook 1A
Workbook 1A
Notebook (and folder)
Before lessons start
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Desks in good order!
No rubbish around!
MATHEMATICS NEARBY
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In World Cup 2002, the total number of goals
is 161. There are 64 matches in total. What is
the average number of goals per match?
By calculator, we have
161  64 = 2.515625
What would you say?
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2.515625
2.515
2.5
ESTIMATION
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An estimation is a rough guess on the actual
values.
It does not represent the real answer.
In real life, we seldom bring a calculator with
us. So, we can use estimation to make some
decisions.
EXAMPLE
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In a restaurant, your father ordered a set
dinner costing $38, your mother ordered a
chef salad costing $23 and you ordered
grilled ribs costing $48. How much does your
family have to pay for this meal?
$38 + $23 + $48
 $40 + $20 + $50 = $110
Hence, your family has to pay about $110.
Actually, the answer is $109! (very close!)
ROUNDING OFF A NUMBER (四捨五入)
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To get a better estimation, we need some
techniques to round off a number.
Consider the “unwanted” leftmost digit.
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If greater or equal to 5, add 1 to left digit next to it.
Otherwise, leave left digit next to it unchanged.
E.g. 21.29 + 15.78 + 30.97 + 3.24 + 5.86
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Rounding off to nearest tenth digit
Rounding off to nearest unit
Rounding off to nearest ten digit
Ronald HUI
Enjoy the world of Mathematics!