Since the denominators are not the same, the first step was to find the least common denominator (LCD). When we add numerical fractions, once we find the LCD, we rewrite each fraction as an equivalent...Since the denominators are not the same, the first step was to find the least common denominator (LCD). When we add numerical fractions, once we find the LCD, we rewrite each fraction as an equivalent fraction with the LCD. In the example above, we must leave the first rational expression as 2a(2a−b)b(2a+b)(2a−b) to be able to add it to 3a·b(2a+b)(2a−b)·b.