Читать книгу SAT Math For Dummies with Online Practice - Mark Zegarelli - Страница 47

Multiplying and dividing fractions is often easier than adding or subtracting them, because you don’t need to find a common denominator.

To multiply two or more fractions, first multiply the numerators across to find the numerator of the answer, then multiply the denominators across to find the denominator of the answer. For example:

In some cases, you can make multiplying fractions easier by canceling pairs of equivalent factors in the numerator and denominator. For example, when multiplying and , begin by canceling a factor of 4 in both the numerator and denominator:

Next, cancel a factor of 5:

Now when you multiply, the result is already the most simplified form of the answer:

To divide one fraction by another, change the division to multiplication using the mnemonic Keep-Change-Flip: Keep the first fraction as it is, Change the division sign to a multiplication sign, and Flip the second fraction to its reciprocal. For example:

As when multiplying fractions, you can sometimes make dividing fractions easier by canceling common factors in both the numerator and denominator. Be careful to cancel factors after changing the division to multiplication. For example, when dividing by , begin by using Keep-Change-Flip:

Now, cancel a common factor of 2 in both the numerator and denominator:

To complete the problem, multiply: