In many mathematical expressions an unknown term is used. Often this term is called #x#, but any other name (such as #y# or #F#) which has no other meaning (which can lead to confusion) will work as well.

This unknown term can be used in a particular law or formula, which shows the relationship between two things. Let's consider the law that expresses the relationship between temperature in degrees of the earlier mentioned Celsius and Fahrenheit scales:

\[C = \frac{5}{9}(F-32)\tiny,\]

Here #C# is the temperature in degrees Celsius and #F# the temperature in degrees Fahrenheit.

It depends on the situation if #C# is the unknown or #F#. Here you see the linear relationship between #C# and #F# in a graph.

This unknown can adhere to a specific law.

It depends on the situation which of these two is the unknown: #C# or #F#.

If we already know the value of #F#, then this law tells us exactly what the corresponding value of #C# is. 
In other words: #C# is a function of #F#.

It's even a linear function of #F#, because of the form #aF+b# with #a =  \dfrac{5}{9}# and #b =  -\dfrac{160}{9}#.

If the value of #C# is known, let's say #20#, then we can find the corresponding value of #F# by solving the linear equation \[20 = \dfrac{5}{9}F-\dfrac{160}{32}.\]

The purpose of this chapter is to learn how to solve linear equations with one unknown.

We will also treat some types of equations which can be reduced to linear equations (inverses, quotients in two linear functions and absolute values).

We will also see other applications such as interest on loans, participation in stock options, and the decrease of resistance of electrical connections.