Math basics › Numbers and fractions
Fractions
Adding needs a common denominator
a/b + c/d = (ad + cb)/(bd). Multiplying does not: a/b × c/d = ac/bd, straight across.
Dividing is multiplying by the flip
a/b ÷ c/d = a/b × d/c. “Keep, change, flip.”
Cancelling
You may cancel a factor of the whole top and the whole bottom, never one term out of a sum. (x + 2)/2 is not x.