18
The common denominator is usually the Lowest (or Least) Common Multiple of the numbers involved. Where two numbers have no common factors then the LCM is simply the product of those numbers. As 19 is a prime number and has no factors in common with 18 then the common denominator is 18 x 19 = 342.
The common denominator of 9 and 13 is 1
A common denominator is 72.
The least common denominator of 3 and 13 is 39.
The least common denominator of 13 and 52 is52
The lowest common multiple, or lowest common denominator, of 18 and 28 is 252.
A common denominator, though the least, is 260.
A common denominator, though not the least, is 5616.
18
The LCM is 52
If 13 and 518 are both denominators, the LCD is 6734 If that's 1/3 and 5/18, the LCD is 18 and you can change 1/3 to 6/18