When adding or subtracting two fractions with denominators that are not equal, what is the first step you take?

what would you do if you didn’t know what the values of the denominators were, such as when there are variables in the denominators? for example, what denominator would you use if you were adding 1/a and 1/b? how would your new denominator affect the numerators?

can you come up with denominators that contain variables and aren’t equal that would require you to change only one denominator? explain.