In order to add and subtract using non-common denominators, you must find a common denominator. The easiest way of finding one is to multiply the two denominators. But there may be a smaller number (least common denominator).
Mutiply the numerator and denominator of the first number by the second denominator (effectively multiplying by 1). Then multiply the numerator and denominator of the second number by the original denominator of the first number. This will leave the denominators the same.
Example : add 1/3 and 1/4
Multiply 1/3 by 4/4 to yield 4/12. Multiply 1/4 by 3/3 to yield 3/12. Add the numerators.
1/3 + 1/4 = 4/12 + 3/12 = 7/12
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Because to add fractions, the denominators must be equal.
The same way you would add or subtract whole numbers, leaving the denominators alone.
The denominators must be the same before you can add or subtract fractions.
When you add or subtract fractions with like denominators, you just add or subtract the numerators and put them over the same denominator. For example, 3/5 - 2/5 = (3 - 2)/5 = 1/5
It stays the same. Only the numerators change.