When we first learn about common denominators, we use the idea of the LCM, or least common multiple. For example, if you were asked to do 1/2 - 1/6, a common denominator that would work is 12, but it's not the least common denominator (which would be 6). Using the least common denominator simplifies our work later. Here, we apply that idea to finding the LCM of monomials, binomials, and trinomials: we could technically multiply them to find a multiple that would work, but often, the least common multiple will make our work easier in the long run.
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