Example1.1.7Adding One Number of Each Sign
Here are four examples of addition where one number is positive and the other is negative.

\(15+12\)
We have one number of each sign, with sizes \(15\) and \(12\text{.}\) Their difference is \(3\text{.}\) But of the two numbers, the negative number dominated. So the result from adding these is \(3\text{:}\)
\begin{equation*} 15+12=3 \end{equation*} 
\(200+(100)\)
We have one number of each sign, with sizes \(200\) and \(100\text{.}\) Their difference is \(100\text{.}\) But of the two numbers, the positive number dominated. So the result from adding these is \(100\text{:}\)
\begin{equation*} 200+(100)=100 \end{equation*} 
\(12.8+(20)\)
We have one number of each sign, with sizes \(12.8\) and \(20\text{.}\) Their difference is \(7.2\text{.}\) But of the two numbers, the negative number dominated. So the result from adding these is \(7.2\text{:}\)
\begin{equation*} 12.8+(20)=7.2 \end{equation*} 
\(87.3+87.3\)
We have one number of each sign, both with size \(87.3\text{.}\) The opposing forces cancel each other, leaving a result of \(0\text{:}\)
\begin{equation*} 87.3+87.3=0 \end{equation*}