The reason why is that a fraction is a different numbering system than whole numbers. The "whole number" system is based on units of 100. But a fraction is only a part of a whole number, so in order to add or subtract you must make both fractions the same type of unit just like whole numbers. You cannot add 1/6 and 2/24 unless you have the same units...which in this case is 24. So, multiplying the six by 4 will give you 4/24, and now you can add the numerators together because they are in the same "family".
like, common denominator
Fractions can only be added or subtracted if the denominators are the same. If the denominators are different, then the fractions need to be made into equivalent fractions with the same denominator. The new denominator can be found simply by multiplying the denominators together, but this can lead to some large fractions with which to work. A better new denominator is the lowest common multiple of (all the) denominators. (Once the new denominator is found, the fractions' new numerators are found by multiplying their current numerator by the new denominator divided by their current denominator to make their equivalent fractions with the new denominator.) Once all the fractions are converted into equivalent fractions with the new denominator then the fractions can be added or subtracted, with the result being simplified (if possible).
Like Fractions
You DO need a common denominator to add, subtract, or compare fractions. You DO NOT need a common denominator to multiply or divide fractions.
Addition or subtraction of fractions require "like" fractions: that is, fractions with the same denominator.
like, common denominator
That simply means that the fractions have different denominators. Such fractions cannot be directly added, subtracted, or compared; you have to convert them to a common denominator first.
LCD is the Lowest Common Denominator which is the lowest number into which the denominators of all the fractions will each divide exactly. Each of the fractions can then be converted to an equivalent fraction with the new denominator which then allows the fractions to be added and/or subtracted.
Fractions can only be added or subtracted if the denominators are the same. If the denominators are different, then the fractions need to be made into equivalent fractions with the same denominator. The new denominator can be found simply by multiplying the denominators together, but this can lead to some large fractions with which to work. A better new denominator is the lowest common multiple of (all the) denominators. (Once the new denominator is found, the fractions' new numerators are found by multiplying their current numerator by the new denominator divided by their current denominator to make their equivalent fractions with the new denominator.) Once all the fractions are converted into equivalent fractions with the new denominator then the fractions can be added or subtracted, with the result being simplified (if possible).
You don't need a common denominator to divide fractions.
The least common denominator is the least common multiple of the denominators of the fractions.
Like Fractions
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.
By finding the lowest common denominator of the fractions.
No only when adding or subtracting fractions a common denominator is needed
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You DO need a common denominator to add, subtract, or compare fractions. You DO NOT need a common denominator to multiply or divide fractions.