Form 1 Mathematics Chapter 5

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

 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.

 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  Rounding off to nearest tenth digit ( 十分位 )  Rounding off to nearest unit ( 個位 )  Rounding off to nearest ten digit ( 十位 )

 Rounding off to nearest tenth digit   = 77.2  Rounding off to nearest unit (or one)   = 77  Rounding off to nearest ten digit   = 80  The actual answer is 77.14

 Consider the leftmost digit.  Leave the first leftmost digit  Other change to 0!  E.g   = = 68(Actual answer is 77.14)

 Consider the “unwanted” digits.  All change to 0  Add 1 to left digit next to them  E.g  Rounding up to nearest tenth digit  Rounding up to nearest unit  Rounding up to nearest ten digit

 Rounding up to nearest tenth digit   = 77.3  Rounding up to nearest unit (or one)   = 79  Rounding up to nearest ten digit   = 110  Actual answer = All are over-estimated.

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

 Rounding down to nearest tenth digit   = 76.8  Rounding down to nearest unit (or one)   = 74  Rounding down to nearest ten digit   = 60  Actual answer = All are under-estimated.

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

 Choose one approximate value which is close in value to all (or most) numbers  E.g  x 6 =

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

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

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

 Page 200 of Textbook 1A  Questions 7 – 12  Page 94 of WB 1A  Questions  SHW (I)  Questions

 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

Enjoy the world of Mathematics! Ronald HUI