And A = 1/B => AB = 1.
It is the same as multiplying (or dividing) both sides by B (or 1/B), which is also the same thing as multipling one side by <something> divided by <the same thing> which equals 1 so you don't change the equation, and then moving <the same thing> to the other side so it changes from divide to multiply. THe result is multipling both sides by the same number which originates from multipying one of the sides by 1, so nothing changes.
Another way to think about it is when something is moved from one side of the equal sign to the other, it's operation becomes reversed:
addition <-> subtract
multiply <-> divide
or pretty much any other operation...trig, log, whatever. THough these don't quite work in the same way.
Also,
1/(A/B) = B/A
Why is this? It's the same as multipling both the numerator AND denominator by (1/B). THe result is that you are multipling the everything on the left-side of the equal sign by
(1/B)/(1/B) = 1, so it doesn't change the equation. It just makes it look different.
Does that answer your question? Because honestly, I don't understand the equation you just wrote probably because of the format it is in.
So start with the fractions that are most nested and rewrite them so they are easier to read, and work your way up. Start with the 1, 9, and (A-4). Rewrite that in the entire equation first.
It all comes down to multiplying various things in the equation by an expression that is equal to 1 (which almost always comes in the form of a fraction that has the same numerator and denominator which matches something other part of your equation). The first key parts of equations to multiply by 1 are both the numerator and denominators pair that appears anywhere in the equation. THe second is one side of the equation and then moving the denominator of the "equal to 1" expression the other side (this is the same as multipling both sides of an equation by the same thing, because you are fundamentally just multipling one side by 1).