Too many boxes in the illustrations
The diagram at the beginning is difficult to read. Rather than having boxes (or "callouts") with a, b, and a + n, with curved lines going over to the two vertical lines, it would be better to have two horizontal lines of equal length, with one of them divided into two parts. Put a label of b on the undivided line, and put labels of a and a + n on the divided line. No boxes, no curved lines.
The second figure is even harder to understand, due to the boxes that represent the quantities (a + n)
2, a
2, and b
2/2, as well as the other boxes that clutter up the drawing.
Equation numbering
Examples such as the one shown below confused me at first.
As shown, it looked to me like the left side was 1 - b, not b as you intended.
The usual practice when equations are numbered is to put the number after the equation, in parentheses, like this:
b = a + (a + n)
(1)
I wrote the parenthesized 1 in italics so as to not be interpreted as multiplication by 1.
Also, when equations are shown with numbers, usually only the more important ones are numbered, when they are discussed later in some detail. You shouldn't number steps that aren't equations, such as when you multiply both sides by b
k-2.