The cubing of a binomial is defined as the multiplication of a binomial 3 times itself. The sum of binomial expression is in the form of a + b. So, the cube of the binomial is expressed as (a + b) (a + b) (a + b) = (a + b)³. Therefore, the rule for cubing a binomial should be either in the form of a³ + b³ or (a + b)³. Its formula is (a + b)³ = a³ + b³ + 3ab.