Download 第二學習階段

Key Stage 3
Number and Number Systems
Learning Unit : Numerical Estimation
Learning Objectives :
· Be aware of the need to use estimation strategies in real-life
situations and appreciate the past attempts to approximate
values such as π
· determine whether to estimate values or to compute the exact
values
· select and use estimation strategies to estimate values and to
judge the reasonableness of results
· choose appropriate means for calculation such as mental
computation, calculators or paper and pencil etc.
Programme Title : Numerical Estimation
Programme Objectives
1. Use examples to demonstrate that in solving problems, sometimes it is more
effective to use estimate values in stead of to compute the exact values.
2. Make use of some real-life situations to illustrate the meaning of estimation.
3. Introduce different estimation strategies.
4. Introduce the concept of ‘rounding off with over-estimates’ and ‘rounding
off with under-estimates’.
Programme Outline
In comparing the prices of two different packing of dry beef, the idea of using
estimated values to solve problems in real-life situations is introduced.
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With the help of some real-life examples, the benefits of using estimation and
different estimation strategies, such as using compatible numbers, number
re-arranging, mid-value method, value adjusting method and changing problem
structure method etc., are introduced.
The concept of ‘rounding off with over-estimates’ and ‘rounding off with
under-estimates’ is illustrated through a context on a bargain at a vegetable stall.
Worksheet Answers
1.
(a) Use rounding off method, expression  3020600；
use changing problem structure method,
100
expression
241006600
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(b) Use rounding off method, expression
use compatible numbers, expression
 61.8302.03；
 62312
(c) Use mid-value method, expression  24051200；
use value adjusting method,
expression  2005(284131) (5546) 1200
2.
The area of the rectangle lies between 10cm9cm and 11cm10cm, i.e.
between 90cm2 and 110cm2, therefore area of rectangle  100 cm2.
3.
Rounding off with over-estimates. To ensure sufficient storing capacity
for keeping all files to be stored.
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Key Stage 3 ETV programme《Numerical Estimation》
Worksheet
1. Use different methods to estimate the values of the following expressions:
(a) 25 × 24
(b) 61.8 ÷ 31
(c) 228 + 255 + 242 + 231 + 246
2.
Estimate the area of the rectangle, in cm2.
10.83cm
9.58cm
3. You need to store some electronic files in a floppy disk and all the file sizes
are different. In order to consider whether all these files can be stored in
the floppy disk, which method, “Rounding Off With Over-estimates” or
“Rounding Off with Under-estimates”, would you use to estimate the
overall file size of the electronic files? Why?
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