This is basic algebra.
The kinds of rules that we have been discussing are generalizations about numbers. If you don't understand a mechanical rule, try it with numbers (generally not 0 or 1 because they have some special properties.
\(\displaystyle \dfrac{6 + 2}{2} = \dfrac{8}{2} = 4.\) Agree?
\(\displaystyle \dfrac{6 + 2}{2} = \dfrac{6 + 1}{1} = 7.\) WRONG, WRONG, WRONG.
\(\displaystyle \dfrac{6 + 2}{2} = \dfrac{2(3 + 1)}{2} = \dfrac{3 + 1}{1} = 4.\) Perfectly OK.
A rule of algebra is
\(\displaystyle \dfrac{ab + ac}{ad + ae} \equiv \dfrac{a(b + c)}{a(d + e)} \equiv \dfrac{b + c}{d + e}.\)
If you study foundations of mathematics, you can prove that rule. But if you want to understand why it works, do some examples.