Because you can't add fractions unless they have the same denominator.
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When you're adding or subtracting fractions with different denominators, if you multiply the denominators that's your Quick Common Denominator. I'ts useful for denominators that don't have a low Least Common Factor.
When adding or subtracting fractions with different denominators the lowest common denominator is needed and is found by finding the lowest common multiple of the denominators.
If the denominators are not the same, then you have to use equivalent fractions which do have a common denominator . To do this, you need to find the least common multiple (LCM) of the two denominators. To add fractions with unlike denominators, rename the fractions with a common denominator.
When adding or subtacting fractions make sure that the denominators are of the same values if they are not then find the lowest common denominator.
Subtracting fractions is similar to adding fractions. If the fractions have the same denominator, you subtract the numerators. If the fractions have different denominators, you have to convert to a common denominator first.Subtracting fractions is similar to adding fractions. If the fractions have the same denominator, you subtract the numerators. If the fractions have different denominators, you have to convert to a common denominator first.Subtracting fractions is similar to adding fractions. If the fractions have the same denominator, you subtract the numerators. If the fractions have different denominators, you have to convert to a common denominator first.Subtracting fractions is similar to adding fractions. If the fractions have the same denominator, you subtract the numerators. If the fractions have different denominators, you have to convert to a common denominator first.