Common Denominators - Fractions - What is the lowest common denominator?

# Common Denominators

If you want to add two fractions together, both fractions must have the same denominator. That's okay if you want to add 1/9 and 4/9 = 5/9. But what if you need to add 1/9 and 4/7? How can you add them? You need to change one or both of those fractions so that they have the same denominator.

How can you get 1/9 and 4/7 to both have the same denominator? You need to find a number that has both 9 and 7 as factors.

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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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