We can write this as:

   *A b/ |C*  | a\ | c   *B

The classical version one learns in school:

The lower bound version:

This is intuitive because the large value for $a - b$ is attained when $b = 0$. (since lengths are non-negative, we have $b \geq 0$. if $b = 0$, then the point $A = C$ and thus $a = CB = AB = c$.

A/C (b=0)|| a=c|B

Otherwise, $b$ will have some length that will cover $a$ (at worst), or cancel $a$ (at best). The two cases are something like:

 A ||b ||c|*C ||a || || B

In this case, it's clear that $a - b < c$ (since $a < c$) and $a + b = c$. In the other case, we will have:

 Cb|| || A| || ||ac|| || || || B

Where we get $a - b = c$, and $c < a + b$. These are the extremes when the triangle has zero thickness. In general, because the points are spread out, when we project everything on the $AB=c$ line, we will get less-than( <=) instead of equals ( =).