For the first question, the first step to take when adding or subtracting two
fractions with denominators that are not equal is to determine their least
common denominator (LCD). This is the lowest number that is a multiple of both
denominators.
For the second question, if only variables are given, you can simply multiply
the two variables to obtain the LCD. However, take note that you should multiply
the numerator and the denominator with the variable that would result to the same
LCD for both terms. For example,
(1/a)*(b/b) + (1/b)(a/a) = (b/ab) + (a/ab) = (a + b)/ab
For the third question, yes you can come up with denominators that contain variables that aren’t equal as shown in the example above.