Answer:
1) ok, we know that if we divide one number by the same number, the solution is 1 ; this is: a/a = 1
so we could write something as:
3 = 3*1 = 3*(5/5) = (3*5)/5 = 15/5
so 3 = 15/5
this is especially useful when working with addition of fractions.
2) this is really useful if you have a square root in the denominator.
for example :
3) you also could do this in the next way:
3 = 3*1 = 3*(7/7) = 3*(3+4)/7= 9/7 + 12/7
where i splitted the 7 into 3 +4, this relation could be useful sometime.
4) if you do the difference between some number and the prior number; you get one; this is : (5 -4) =1
then: 3= 3*1 = 3*(5 - 4) = 15 - 12