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# Drawing linear functions - what are linear functions?

## What are Linear Functions?

Linear functions explanation - What are linear functions?

To do this, we start with assignments. Then we get into the concept of a function and then we move on to the linear function.

Mathefritz tries to explain the linear function as simply and precisely as possible. You can also find more theoretical information on the topic of "linear function" at Wikipedia!

## Assignments - Entry

How do linear functions work - to understand this, we first need to look at mappings.

### Examples of assignments in everyday life

Example 1:
Peter buys walnuts at the fruit shop. One kilogram costs 4 €. How much do 200 g, 500g, 1.2 kg cost?

Example 2:
In a multi-storey car park, parking costs €1 per half hour or part thereof. From 2 hours, the half hour costs only €0.75. Parking for longer than 4 hours in one day costs a flat rate of €7.50 (i.e. for the whole day).
Calculate the parking fee for 10 minutes, for 50 minutes, for 2 hours 20 minutes, for 6 hours 15 minutes.

Example 3:
The Meyers family has 4-minute eggs for breakfast every Sunday.
How long does it take to cook 2, 3 or 5 eggs?

Notice:
In these examples, values are always assigned to each other:

• The quantity of walnuts is assigned the price for them.
• A parking fee is assigned to the parking duration.
• The eggs are assigned a cooking time.

In order to present such assignments more clearly and to answer the questions in the examples, there are various possibilities. One of them is the value table.

### Value tables - on the way to linear functions

Our example 1:

 kg walnuts 1 kg = 1000g 100 g 200 g 500 g = 0.5 kg 1.2 kg =1200 g Price in € 4 € 0,40 € 0,80 € 2,00 € 4,80 €

In order to easily calculate the prices for 200 g, 500 g and 1.2 kg, it is helpful to first note the price for a small quantity, here for example for 100 g. This is already an example of a table of values for linear functions.

Example 2:

 Parking time 0.5 h = 30 min 10 min 50 min 2h 20min 6h 15min Parking fee 1 € 1 € 1€+1€ = 2 € 4 x 1€ + 0,75€ = 4,75 € 7,50 €

In order to calculate the parking fees, it should be noted that they do not change evenly, but "jump".

Example 3:

 Number of eggs 1 Egg 2 eggs 3 eggs 5 eggs 10 eggs Cooking time in min 4 min 4 min 4 min 4 min 4 min

All eggs are ready at the same time, because all eggs have the same time to be cooked.

### Diagrams

The assignments can also be represented graphically in a chart. To do this, the values are drawn in a coordinate system. You can then also read off other values.

The walnut example - our example 1 as a diagram

What does 800 g of walnuts cost? This graphical allocation is already a linear function as a chart! 800 g walnuts cost €3.20!

The car park example - Example 2 as a diagram

What is the price in the parking garage for 1:15 h parking? 1:15 parking costs €3!

Cooking eggs - Our example 3 Also 8 eggs have exactly 4 minutes cooking time!

Such assignments are also called functions. With functions, exactly oney value (e.g. y €) is assigned to a certain x value (e.g. x kg).

In a graph , functions are represented by corresponding lines. These are called the graph of the function.

## Linear functions Slope

### linear function - the slope

In the example with the walnuts you can see: If twice as many nuts
$$2 \cdot x$$ kg
are bought, then they cost twice as much
$$2 \cdot y$$  € .
Such a function is called a proportional function. It is represented in the diagram by a
Straight which runs through the origin of the coordinate system. This straight line can be represented by the Equation:
$$y = m \cdot x$$ kg
describe. Thereby means the Gradient factor or simply Slope.
It is calculated by transforming the equation:
$$m = \frac{y}{x}$$ .
The slope factor is the measure of the Slope of the straight line in the chart.
Example calculation:In example 1 (see above, nuts) one takes from the value table e.g.   x = 1000 g and y = 4 €.This gives the slope to:
$$m = \frac{y}{x}=\frac{4}{1000}=\frac{1}{250} =0,004$$ .
So the equation holds:
$$y = 0,004 \cdot x$$ .
The solution is then calculated in €.Let us take the following calculation example: What does 600 g of nuts cost?Then we have to do the math:
$$y = 0,004 \cdot 600 = 2,4$$
So the answer is: 600 g of nuts cost €2.40.Summary - Remember:To determine the equation of the straight line of a proportional function, one can calculate the gradient factor with assigned values from the value table.

### The gradient triangle - An example

In the mountains, you will often find road signs indicating the gradient or slope of the road. For example, 10 % means that a road rises by 10 m for 100 m of horizontal distance.

In the diagram of a proportional function, one can determine the slope factor or the slope with a slope triangle.

$$m = \frac{10}{100}=0,1$$
and the straight line equation is
$$y = 0,1 \cdot x$$ .
The gradient of a road is 15 %.So it goes downhill by 15 m if you travel 100 metres horizontally. Thus the y-value is negative: - 15.So is
$$m = \frac{-15}{100}= -0,15$$
and the equation is:
$$y = -0,15 \cdot x$$.
If one knows the equation of the straight line, one can draw the diagram of the straight line with the help of a Gradient triangledraw.

### Further examples - gradient and graph of the linear function

So you can draw linear functions if you know the slope.

Example 1:

$$y= -\frac{1}{2}\cdot x$$

The slope in this functional equation

$$m = -\frac{1}{2}$$

means: go 2 to the right and 1 down. Downwards because of the minus sign -1!

The straight line goes through the points (0;0) and (2;-1).

Example 2:

$$y= 3 \cdot x$$

Go 1 to the right and 3 up to the point (1;3).

## The definition of a linear function

### Example for introduction

With her electric bike, Nikki can ride 40 km on one battery charge. If she rides smoothly, she will have used a quarter of the battery's charge after 10 km, so she still has 30 km "reserve".

To draw the diagram for this, we first create a table of values:

 x: Battery consumption 0 1/4 1/2 3/4 1 y: possible distance 40 km 30 km 20 km 10 km 0 km

The points all lie on a straight line.

$$m=-\frac{10}{0,25}=-40$$

The straight line does not go through the origin of the coordinate system, but is shifted upwards by 40. This results in the equation of a straight line:

$$y = -40 \cdot x + 40$$

### Definition linear function

The mathematical definition:

The function

$$f(x): x \mapsto m \cdot x + n$$

is called a linear function with

$$m, n \in \mathbb{R} .$$

Every linear function has a straight line as a graph with the slope m and the y-axis intercept n.

To the linear function f(x) belongs the function equation

$$f(x) = m\cdot x + n$$

or also:

$$y = m\cdot x + n$$

Or the short version:

A function with a straight line equation of the form:

f(x) = m∙x+n

is called a linear function.

### Did you get it - test your knowledge!

Fill in the gaps with the correct answers!
The picture below belongs to the questions! ## Draw linear functions online

### How do you draw linear functions?

If you know the slope of the straight line (linear function) and the y-intercept (the value when passing through the y-axis), you can draw the graph of the linear function.

We name the slope m and the y-axis intercept n.

Try changing the parameters for the slope m and the y-axis intercept n in the interactive window below. Look closely at how the course of the straight line changes.

• m is the slope of the linear function (straight line).
• n is the y-axis intercept of the line, which is the distance on the y-axis at the point of intersection from the origin
• f: y = m x + n is the function equation, the equation of the linear function!

Tutorial - Draw linear functions:

• Use the slider for the slope and the slider for the y-axis intercept.
• Change the values and follow how the equation for the function f changes in the line below.
• You can use the mouse (or your finger on a tablet computer or smartphone) to move and zoom the displayed area.