Hi again Sophie,
I'd like to try and explain some stuff. :)
FIRST a fraction sign IS a division sign. It even looks like a division sign!
Think of the numbers on the top and bottom as dots and you have the stroke in the middle - that is a division sign
also
Letters just take the place of numbers so if you can do somethng with numbers then you can do it with letters too.
also
\(\frac{3}{4}=\frac{1+2}{4}=\frac{1}{4}+\frac{2}{4}\\ so\\ \frac{1+a}{4}=\frac{1}{4}+\frac{a}{4}\\\)
etc
If you are not sure if you are allowed to do something with letters, replace those letters with numbers.
Now maybe you can see what you can or cannot do...
I hope that helps :))