For example, if the fractions of two different denominators are added and the denominators of two numbers are directly multiplied, then the number obtained must be the common multiple of these two denominators. As for the numerator, multiply the numerator of the first number by the denominator of the second number, and the numerator of the second number by the denominator of the first number. In this way, two numbers with different denominators are replaced by numbers with the same denominator. No matter how many fractions of different denominators are added up, they can be calculated in this way, and then simplified after direct addition.