1TH/Vwo §2.4 Decimal numbers

• Wiskunde
• Middelbare school
• havo, vwo
• Leerjaar 1

§ 2.4 Decimal numbers

Some important points follow now!

Fractions  -  decimal numbers

These are closely linked together!

We can write a fraction as a decimal number:

= 0.8,   or 

                                                             = 0.75

​5​​4​​

​4​​3​​

In order to write a fraction as a decimal number,

 we first have to write the fraction with a denominator of 10, 100 or even 1000!

For example:

5                =    5                 =  5.85                                 

             =               =                = 0.175   Now try yourself!

​20​​17​​

​40​​7​​

​100​​85​​

​200​​35​​

​1000​​175​​

​100​​85​​

​30​​6​​

Task 1: Write as a decimal number:

Task 1  Explanation:

We have to have a number like 10, 100 or 1000 as a denominator!

=                         =                   =   0.2        

​30​​6​​

​5​​1​​

​10​​2​​

write as a decimal number:

​250​​3​​

1. 0.12
2. 0.012
3. 1.2
4. 0.0012

Task 2  Explanation:

Now we even need to have 1000 as a denominator!

=                         =       0.012               

​250​​3​​

​1000​​12​​

Rounding off can give us a better overview:

For example:

If you read in the papers that 

'298 785 sheep were transported,

it makes sense to round this off:

'About 300 thousand sheep were transported.'

Rounding off ... (1)

see next slide how to do this!

Rounding off ... (2)

Your marks are also rounded off to one decimal place at the end of the year:

+      7.79 for French become a 7.8 on your report

+      5.73 for History becomes a 5.7 on your report

How does this work? Let's have a closer look!

round to two decimal places:
4.7386

1. 4.73
2. 5.74
3. 4.74
4. 473

4.73 86

Rule for rounding off:

If the digit to the right of 3 is '5 or higher', then 3 is made one higher.

In this case: 8 is certainly '5 or higher', so the rounded number is:  4.74

So you see:   the 3 turns into a 4!

4.7386    .......

becomes 4.74,

because the '8', after the '3', is higher than 4. 

Task 3: Round to two decimal places:
79.6754

This is how it should've been done:

79.6754                      79.68

Because 5 is '4 or more',  the 7 is made one higher!

≈

round to three decimal places:
3.670587

1. 3.670
2. 3.671
3. 3.67

Round to two decimal places:
93.098

1. 94
2. 93.09
3. 93.10

Round to two decimal places:

 73.098

This one is a bit tricky!

Between the 9 and the 8 a dotted line is drawn.

Because the 8 is higher than a 4, the previous digit, 9, is

made one higher. So we write down '10', as follows:

73.10

Homework time

+ now do §2.4 Decimal numbers

+ check your work with the Solutionsbook

+ good luck!