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When adding or subtracting fractions with different denominators finding the prime product of each denominator helps in finding the lowest common denominator of the given fractions by their lowest common multiple.
LCD, in arithmetic, is the lowest common denominator. Given a set of rational fractions, it is the smallest integer which each of the denominators will go into evenly.
Given a set of integers, their least common denominator is the smallest positive integer which is a multiple of each member of the set. The definition is extended to fractions where the LCD refers to the LCD of their denominators.
By finding their lowest common denominator and then alter the numerator and denominator of the given fractions accordingly
Make them into equivalent fractions with the same denominator, then the numerators can be subtracted and the resultant fraction simplified (if possible).The first choice is the denominator for the equivalent fractions. Ideally this should be the lowest common multiple (LCM) of the given denominators, but just multiplying the two given denominators together will do.The fractions can then be made into equivalent fractions with this chosen denominator.If finding the LCM is too difficult, a simple "butterfly" (or "cross multiply") method can be used:multiply the numerator of the first fraction by the denominator of the second fraction as the numerator of the equivalent first fraction;multiply the denominator of the first fraction by the numerator of the second fraction as the numerator of the equivalent second fraction;multiply the denominators together as the denominator of both of the equivalent fractions;subtract the new numerators, keeping the new denominator;simplify the result if possible.eg 3/5 - 1/2 = 3x2/5x2 - 5x1/5x2= 6/10 - 5/10= 1/10eg 5/6 - 1/2 = 5x2/6x2 - 6x1/6x2= 10/12 - 6/12= 4/12= 1x4/3x4= 1/3The same method is used for addition of fractions with different denominators: add instead of subtract the numerators of the equivalent fractions.