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You find the least common multiple of the denominators [or least common denominator].

Basically, the Least Common Denominator equals the two denominators multiplied together and then divided by any common prime factors. It is easy to see that the numbers 2 and 3 have a common denominator of 6. But the number 4 and 6 do not have a least common denominator of 24, because both numbers are factors of the number 12 (the least number).

Example : add 1/2 and 1/3 -- multiply the numerator and denominator by the same number : (1/2) x (3/3) = 3/6 and (1/3) x (2/2) = 2/6. We are multiplying them by a fraction equal to 1.

Now you can add 3/6 + 2/6 = 5/6

Example : subtract 1/6 from 3/4 -- multiply 1/6 by 2/2 and multiply 3/4 by 3/3

1/6 = 2/12 and 3/4 = 9/12

so 3/4 - 1/6 equals 9/12 - 2/12 = 7/12

If the result is a reducible fraction, reduce it.

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15y ago

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