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Mathematics I (Math 1132)

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Durham College, Mathematics
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Mathematics I (Math 1132)

Add and Subtract Fractions

Adding and subtracting fractions can get very challenging depending at what level of math you’re studying, but the same principles you learned back in elementary school still apply. We’ll start with similar fractions – that’s when the fractions you’re combining have the same denominators. To combine these, you keep the denominator the same and focus on the numerators only. Here are some example:

1   12+92=1+92=10252   3515=315=25

To best way to combine fractions with different bottoms is to find the lowest common denominator (LCD). Once that’s found, you multiply the numerator and denominator of each fraction by a quantity that will make the denominator of each equal to the LCD. This will ensure that the fractions are similar fractions, which you can combine the same way as above, and simplify.

  • If you happen to find a common denominator instead of the LCD, you’ll end up with a fraction that’s not in lowest terms. For this reason, more simplification would be required at the end.

The video below shows two methods on how to find the LCD of any non-algebraic fraction. Algebraic fractions will be the focus in the unit on algebra.

Putting all these ideas together, the video below shows six examples of fractions being added and subtracted.

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