Step-by-step explanation:
GCF: the greatest common factor of numerator and denominator is a factor that can be removed to reduce the fraction.
Example
The numerator and denominator of 6/8 have GCF of 2:
6/8 = (2·3)/(2·4)
The fraction can be reduced by canceling those factors.
(2·3)/(2·4) = (2/2)·(3/4) = 1·(3/4) = 3/4
___
LCM: the least common multiple of the denominators is suitable as a common denominator. Addition and subtraction are easily performed on the numerators when the denominator is common.
Example
The fractions 2/3 and 1/5 can be added using a common denominator of LCM(3, 5) = 15.
2/3 + 1/5 = 10/15 + 3/15 = (10+3)/15 = 13/15