Yes. You need common denominators if you want to:
You do not need common denominators to multiply or divide fractions. Thus, in the case of fractions, multiplication and division is actually easier than addition and subtraction.
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No. To multiple fractions multiple the numerators together and multiply the denominators together and simplify (by dividing both numerator and denominator of the result by common factors until the only common factor is 1). The denominators only need to be the same when adding or subtracting fractions.
You cannot add or subtract fractions with different denominators. If the denominators are different then you need to work with equivalent fractions.
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.
No only when adding or subtracting fractions a common denominator is needed
Finding a common denominator makes it possible to add two fractions because it allows us to write each fraction as a multiple of a common (usually smaller) fraction. Subtracting fractions works the same way; find a common denominator so that the fractions involved are in the same terms.