When fractions having the same denominator, as 3/10 and 4/10, are added, only the numerators are added, and their sum is then written over the common denominator: 3/10+4/10 = 7/10. Fractions having unlike denominators, e.g., 1/4 and 1/6, must first be converted into fractions having a common denominator, a denominator into which each denominator may be divided, before addition may be performed. In the case of 1/4 and 1/6, for example, the lowest number into which both 4 and 6 are divisible is 12. When both fractions are converted into fractions having this number as a denominator, then 1/4 becomes 3/12, and 1/6 becomes 2/12. The change is accomplished in the same way in both cases—the denominator is divided into the 12 and the numerator is multiplied by the result of this division. The addition then is performed as in the case of fractions having the same denominator: 1/4+1/6 = 3/12+2/12 = 5/12. In subtraction, the numerator and the denominator are subjected to the same preliminary procedure, but then the numerators of the converted fractions are subtracted: 1/4 - 1/6 = 3/12 - 2/12 = 1/12.
In multiplication the numerators of the fractions are multiplied together as are the denominators without needing change: 2/3×3/5 = 6/15. It should be noted that the result, here 6/15, may be reduced to 2/5 by dividing both numerator and denominator by 3. The division of one fraction by another, e.g., 3/5÷1/2, is performed by inverting the divisor and multiplying: 3/5÷1/2 = 3/5×2/1 = 6/5. The same rules apply to the addition, subtraction, multiplication, and division of fractions in which the numerators and denominators are algebraic expressions.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.