1.1 Verschiedene Zahlen
Aus Online Mathematik Brückenkurs 1
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Version vom 16:27, 2. Jul. 2008
Contents:
- Natural numbers
- Negative numbers
- Order of precedence and parenthesis
- Rational numbers
- Briefly about irrational numbers
- Real numbers
Learning outcomes:
After this section, you will have learned to:
- Calculate an expression that contains integers, the four arithmetic operations and parentheses.
- Know the difference between the natural numbers, integers, rational numbers and irrational numbers.
- Convert fractions to decimals, and vice versa.
- Determine which of the two fractions is the larger, either by a decimal expansion or by cross multiplication.
- Determine an approximate value to a decimal number and a fraction to a given number of decimal places.
Calculations with numbers
Calculating with numbers requires you to perform a series of operations. These are the four basic operations of arithmetic. Here are some concepts that are helpful to know in order to understand a mathematical text:
When you add numbers the sum does not depend on the order in which the terms are added together
Vorlage:Fristående formel
As regards subtraction, the order is important of course.
5-2 = 3medan2-5 =- 3. Vorlage:Fristående formel
When we talk about the difference between two numbers we usually mean the difference between the larger and the smaller . Thus, we understand that, the difference between 2 and 5 is 3.
When numbers are multiplied, their order is not important.
Vorlage:Fristående formel
For division the order is of importance.
Hierarchy of arithmetic operations (priority rules)
If several mathematical operations ioccur n a mathematical expression, it is important to have a standard on the order in which the operations are to be carried out. The following rules applies:
- Parentheses ( brackets, "innermost brackets" first)
- Multiplication and Division (from left to right)
- Addition and subtraction (from left to right)
Example 1
- \displaystyle 3-(2\cdot\bbox[#FFEEAA;,1pt]{(3+2)}-5) = 3-(\bbox[#FFEEAA;,1pt]{\vphantom{()}2\cdot 5}-5) = 3-\bbox[#FFEEAA;,1pt]{(10-5)} = 3-5 = -2
- \displaystyle 3-2\cdot\bbox[#FFEEAA;,1pt]{(3+2)}-5 = 3-\bbox[#FFEEAA;,1pt]{\vphantom{()}2\cdot 5}-5 = \bbox[#FFEEAA;,1pt]{\vphantom{()}3-10}-5 = -7-5 = -12
- \displaystyle 5+3\cdot\Bigl(5- \bbox[#FFEEAA;,1pt]{\frac{-4}{2}}\Bigr)-3\cdot(2+ \bbox[#FFEEAA;,1pt]{(2-4)}) = 5+3\cdot\bbox[#FFEEAA;,1pt]{(5-(-2))} -3\cdot\bbox[#FFEEAA;,1pt]{(2+(-2))} \displaystyle \qquad{}=5+3\cdot\bbox[#FFEEAA;,1pt]{(5+2)} -3\cdot\bbox[#FFEEAA;,1pt]{(2-2)} = 5+\bbox[#FFEEAA;,1pt]{\vphantom{()}3\cdot 7} - \bbox[#FFEEAA;,1pt]{\vphantom{()}3\cdot 0} = 5+21-0 = 26
"Invisible" parentheses
For division the numerator and the denominator must be calculated separately before the division is carried out. One can therefore say that there are "invisible parentheses" around the numerator and denominator.
Exempel 2
- \displaystyle \frac{7+5}{2} = \frac{12}{2} = 6
- \displaystyle \frac{6}{1+2} = \frac{6}{3} = 2
- \displaystyle \frac{12+8}{6+4} =\frac{20}{10} = 2
This is especially important if calculators are used.
Division Vorlage:Fristående formel
must be written as\displaystyle (8 + 4 )/(2 + 4) on a calculator so that the correct answer \displaystyle 2 may be otained. A common mistake is to write \displaystyle 8 + 4/2 + 4, which the calculator interprets as \displaystyle 8 + 2 + 4 = 14.
Different types of numbers
The numbers we use to describe the “how many” and size, etc.., are called generically the real numbers and can be illustrated by a straight line real-number axis:
The real numbers "fill" real-number axis:, ie. there are no holes or spaces are along the real-number axis. Each point on the real-number axis can be specified by a decimal. The set of real numbers are all the decimals , and is denoted by “R”. The real-number axis also shows the relative magnitude of numbers; a number to the right is always higher than a number to the left.It is standard to classify the real numbers into the following types:
Natural numbers (usually symbolized by the letter N)
The numbers which are used when we calculate “how many”: 0, 1, 2, 3, 4, ...
Integers (Z)
The natural numbers and their negative counterparts: ..., -3, -2, -1, 0, 1, 2, 3, ...
Rational numbers (Q)
All the numbers that can be written as a ratio of whole numbers (fractions), for example, Vorlage:Fristående formel
Note that even integers count as rational numbers, because real-number axis Vorlage:Fristående formel
A rational number can be written in various ways since, for example,
Vorlage:Fristående formel
Example 3
- Multiplying the numerator and denominator of a rational number with the same factor does not change the value of the number Vorlage:Fristående formel
- Dividing the numerator and denominator of a rational number with the same factor called reducing and does not change the value of the number. Vorlage:Fristående formel
Irrational numbers
The numbers on the real-number axis that can not be written as a fraction are called irrational numbers. Examples of irrational numbers are most roots, for example
\displaystyle \sqrt{2} och \displaystyle \sqrt{3},but also numbers such as \displaystyle \pi
Decimal form
All types of real numbers can be written in decimal form, with an arbitrary number of decimal places. Decimal integers written to the right of the decimal point specify the number of tenths, hundredths, thousandths, and so on., In the same way as the integers to the left of the decimal point indicates the number of units, tens, hundreds, and so on.
Example 4
A rational number can be written in decimal form by performing the division. Thus the
\displaystyle \textstyle\frac{3}{4} is the same as "3 divided by 4", i.e.. 0,75.
Read liggande stolen on wikipedia.
Exempel 5
- \displaystyle \frac{1}{2} = 0{,}5 = 0{,}5\underline{0}
- \displaystyle \frac{1}{3} = 0{,}333333\,\ldots = 0{,}\underline{3}
- \displaystyle \frac{5}{12} = 0{,}4166666\,\ldots = 0{,}41\underline{6}
- \displaystyle \frac{1}{7} =0{,}142857142857\,\ldots = 0{,}\underline{142857}
(underlining signifies that the decimal is repeated)
As can be seen the rational numbers above have a periodic decimal expansion, ie. The decimal expansion, ends up with a finite block of digits that is repeated endlessly. This applies to all rational numbers and distinguishes them from the irrational numbers, which do not have a periodic pattern in their decimal expansion.
Conversely it is also true that all numbers with a periodic decimal expansion are rational.
Example 6
The numbers \displaystyle \pi and\displaystyle \sqrt{2} are irrational and therefore have no periodic patterns in their decimal expansion.
- \displaystyle \pi=3{,}141 \,592 \, 653 \, 589 \,793 \, 238 \, 462 \,643\,\ldots
- \displaystyle \sqrt{2}=1{,}414 \,213 \, 562 \,373 \, 095 \, 048 \, 801 \, 688\,\ldots
Exempel 7
- \displaystyle 0{,}600\,\ldots = 0{,}6 = \frac{6}{10} = \frac{3}{5}
- \displaystyle 0{,}35 = \frac{35}{100} = \frac{7}{20}
- \displaystyle 0{,}0025 = \frac{25}{10\,000} = \frac{1}{400}
Example 8
The number\displaystyle x=0{,}215151515\,\ldots is rational, because it has a periodic decimal expansion. We can write this rational rational number as a ratio of two integers as follows.
Multiply the number by 10 which moves the decimal point one step to the right.
Multiply the number by \displaystyle 10\cdot 10\cdot 10 = 1000 moving the decimal point three steps to the right
Now we see that \displaystyle 1000\,x and \displaystyle 10\,x have the same decimal expansion so the the difference between the numbers
must be an integer.
Vorlage:Fristående formel So that
Rounding off
Since it is impractical to use long decimal expansions so one often rounds off a number to an appropriate number of decimal places. The standard practise is that the numbers 0, 1, 2, 3 and 4 rounded down while 5, 6, 7, 8 and 9 rounded up.
We use the symbol \displaystyle \approx (is approximately equal to) to show that a rounding off has taken place.
Example 9
Rounding off to 3 decimal places:
- \displaystyle 1{,}0004 \approx 1,000
- \displaystyle 0{,}9999 \approx 1{,}000
- \displaystyle 2{,}9994999 \approx 2{,}999
- \displaystyle 2{,}99950 \approx 3{,}000
Example 10
Rounding off to 4 decimal places:
- \displaystyle \pi \approx 3{,}1416
- \displaystyle \frac{2}{3} \approx 0{,}6667
Comparing numbers
To indicate the relative size between numbers one uses the symbols
> (is greater than), < (is less than) och = (is equal to). The relative size between two numbers can be determined either by giving the numbers in decimal form or by representing rational numbers speech as fractions with a common denominator.
Example 11
- Which is greater \displaystyle \frac{1}{3} or \displaystyle 0{,}33?
We have that Vorlage:Fristående formel So \displaystyle x>y as \displaystyle 100/300 > 99/300.
Alternatively, you can see that \displaystyle 1/3>0{,}33 as \displaystyle 1/3 = 0{,}3333\,\ldots > 0{,}33. - What number is the larger \displaystyle \frac{2}{5} och \displaystyle \frac{3}{7}?
Write the numbers with a common denominator, e.g. 35: Vorlage:Fristående formel Thus \displaystyle \frac{3}{7}>\frac{2}{5} as \displaystyle \frac{15}{35} > \frac{14}{35}.
Study advice
'Basic and final tests
After you have read the text and worked through the exercises, you should do the basic and final tests to pass this section. You can find the link to the tests in your student lounge.
Remember
Be careful! Many solutions are wrong because of copying errors or other simple errors, and not because your understanding of the question is wrong.
Reviews
For those of you who want to deepen your studies or need more detailed explanations consider the following references
Learn more about arithmetic in the English Wikipedia
Who discovered zero? Read more in "The MacTutor History of Mathematics archive"
Liggande stolen - a description
Did you know that 0,999... = 1?
Useful links
How many colours are needed to colour a map? How many times does one need to shuffle a deck of cards? What is the greatest prime number? Are there any "lucky numbers"? What is the most beautiful number? Listen to the famous writer and mathematician Simon Singh, who among other things, tells about the magic numbers 4 and 7, about the prime numbers, about Keplers piles and about the concept of zero.
