Function 
Function means relation. If you call, there is a relation to how many minutes you call and how big the cost will be.
The cost is a function of the conversation’s time. 

Formula 
A formula is and algebraic description of a function. To calculate the perimeter of a rectangle, you can also do this with an algebraic description, a formula. The formula looks like this:
Perimeter = base · 2 + height · 2 

Graph 
With a graph, you can show a function in a coordinate system. If you put in a number of points from the function in the coordinate system and connect them, it is called a graph. 

Constant 
A constant is the opposite of a variable. A constant is a number that cannot be replaces by a value. It does not Change. If the temperature is constant, it means that it doesn’t change. It is the same the whole time. 

Coordinate 
A coordinate is a value on the number line. In a coordinate system, a point can have two coordinates, an xvalue and a yvalue. 

Coordinate System 
Consists of two lines, axis. These are called the xaxis and the yaxis. The
xaxis is horizontal and the yaxis vertical. 

Linear Function 
If the graph of a function is a straight line it is called a linear function or a linear relation. If something increases or decreases equally the whole time then this can be described with a linear function.


The Origin 
The point (0, 0) in the coordinate system. It is the point where the xaxis and the yaxis cross, intersect each other. 

Points in the coordinate system 
When we work with functions and formulas, a point is a dot in the coordinate system. The point can have one
xcoordinate and one ycoordinate. (3, 4) is a point which has the value 3 on the xaxis and 4 on the yaxis. 

Proportionality 
Proportionality comes from the word proportion which means relationship. Proportionality describes the relationship between two values.
Example:
2:4 and 6:12. Two relates to 4, as 6 relates to 12.
The graph is a straight line which goes through the origin. 

Gradient 
”The number in front of x”. The gradient in a linear function determines in which direction the line slopes.
Example: y = 2x + 4
This is the “number in front of x”. The 2 determines in which direction the line will slope. 

Variable 
A variable represents an unknown number which can vary, in other words take on different values. In the formula for the mobile phone bill, the call time was a variable. When you know exactly how many minutes you talked, you can replace the variable with a number and calculate the cost for the call. 