1. Form 1 Mathematics Chapter 5

2.  Lesson requirement  Textbook 1A  Workbook 1A  Notebook (and folder)  Before lessons start  Desks in good order!  No rubbish around!

3.  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.  To get a better estimation, we need some techniques to round off a number.

4.  Consider the “unwanted” leftmost digit.  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  Rounding off to nearest tenth digit ( 十分位 )  Rounding off to nearest unit ( 個位 )  Rounding off to nearest ten digit ( 十位 )

5.  Rounding off to nearest tenth digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  21.3 + 15.8 + 31.0 + 3.2 + 5.9 = 77.2  Rounding off to nearest unit (or one)  21.29 + 15.78 + 30.97 + 3.24 + 5.86  21 + 16 + 31 + 3 + 6 = 77  Rounding off to nearest ten digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  20 + 20 + 30 + 0 + 10 = 80  The actual answer is 77.14

6.  Consider the leftmost digit.  Leave the first leftmost digit  Other change to 0!  E.g. 21.29 + 15.78 + 30.97 + 3.24 + 5.86  21.29 + 15.78 + 30.97 + 3.24 + 5.86  20.00 + 10.00 + 30.00 + 3.00 + 5.00 = 20 + 10 + 30 + 3 + 5 = 68(Actual answer is 77.14)

7.  Consider the “unwanted” digits.  All change to 0  Add 1 to left digit next to them  E.g. 21.29 + 15.78 + 30.97 + 3.24 + 5.86  Rounding up to nearest tenth digit  Rounding up to nearest unit  Rounding up to nearest ten digit

8.  Rounding up to nearest tenth digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  21.3 + 15.8 + 31.0 + 3.3 + 5.9 = 77.3  Rounding up to nearest unit (or one)  21.29 + 15.78 + 30.97 + 3.24 + 5.86  22 + 16 + 31 + 4 + 6 = 79  Rounding up to nearest ten digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  30 + 20 + 40 + 10 + 10 = 110  Actual answer =77.14. All are over-estimated.

9.  Consider the “unwanted” digits.  All change to 0  Leave all left digits next to them unchanged  E.g. 21.29 + 15.78 + 30.97 + 3.24 + 5.86  Rounding down to nearest tenth digit  Rounding down to nearest unit  Rounding down to nearest ten digit

10.  Rounding down to nearest tenth digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  21.2 + 15.7 + 30.9 + 3.2 + 5.8 = 76.8  Rounding down to nearest unit (or one)  21.29 + 15.78 + 30.97 + 3.24 + 5.86  21 + 15 + 30 + 3 + 5 = 74  Rounding down to nearest ten digit  21.29 + 15.78 + 30.97 + 3.24 + 5.86  20 + 10 + 30 + 0 + 0 = 60  Actual answer =77.14. All are under-estimated.

11.  Page 195 of Textbook 1A  Class Practice  Page 200 of Textbook 1A  Questions 3 – 6  Pages 93 - 94 of WB 1A  Questions 1 - 4  SHW (I)  Questions 1 - 8

12.  Choose one approximate value which is close in value to all (or most) numbers  E.g. 21356 + 19587 + 22098 + 18657 + 20485 + 19444  20000 x 6 = 120000

13.  Choose a pair of numbers which makes the calculation easier  E.g.345678  9  360000  9 = 40000  E.g.0.2468 x 163  0.25 x 160 = 40

14.  To change the order of the arithmetic operations for easy calculation  Only for first x then   E.g.

15.  Make some adjustments during or after arithmetic operations  E.g.6432 x 7  Consider 6432 x 7 = (6000 +432) x 7  First,6000 x 7 = 42000  Then,432 x 7  3000 (400 x 7 = 2800  432 x 7 is close to 3000)  So, the answer is about 45000

16.  Page 200 of Textbook 1A  Questions 7 – 12  Page 94 of WB 1A  Questions 5 - 6  SHW (I)  Questions 9 - 14

17.  Lesson  Get ready all the time!  Desk  Be order all the time!  Extra Tutorial  Final examination less than 50  SHW (I)  11 Jan (Fri)  OBQ & CBQ  Next week

18. Enjoy the world of Mathematics! Ronald HUI