3.2 Formulas
A function can be written as a formula. When you make a formula, you usually indicate the variables with letters. You may have seen that it is common to use x and y in functions. In formulas, it is more common that other letters are used.
It is good to choose x and y, because mathematicians have called the axis in a coordinate system for the xaxis and yaxis, but you can choose whichever letter you like. We choose to use x and y.
y: total cost of the calls
x: time in minutes the phone has been used
In order to get the total cost of the calls, we look at the time in minutes and multiply it by 2 because every minute costs 2 kr.
In order to get y, we take x and multiply it by 2.
Now we have a formula and can write it mathematically.
y = 2 · x
