Multiplying FractionsDate: 02/06/2001 at 23:22:56 From: Alysha Houston Subject: Multiplying fractions I need help on how to solve this: 8 * 3 - - 9 5 My teacher said something about cross-cancelling, but I didn't understand. Can you please explain? Thanks. Date: 02/07/2001 at 08:47:02 From: Doctor Rick Subject: Re: Multiplying fractions Hi, Alysha. You know the basic method for multiplying fractions, right? You multiply the numerators, and that's the numerator of the product. Multiply the denominators, and that's the denominator of the product. Let's do that, but DON'T do the multiplications yet - just write them where they go. 8 3 8*3 - * - = --- 9 5 9*5 Now, to reduce the fraction to lowest terms, we want to look for a common factor in the numerator and denominator. By not multiplying first, we make this job easier: we have already done some factoring of the numerator and denominator. I'll finish the factoring, but keep the factors together to show where they came from: 8*3 (2*2*2)*3 --- = --------- 9*5 (3*3)*5 Now you can see a common factor: 3. We can "pull them out" of the fraction: 8*3 (2*2*2) 3 --- = ------- * - 9*5 (3)*5 3 But 3/3 = 1, so all that's left is 8*3 (2*2*2) 8 --- = ------- = -- 9*5 (3)*5 15 That's the answer. What we call "cancelling" is really "pulling out" the same number in the numerator and denominator, making a factor of 1, as I did above when I pulled out a factor of 3/3. Now we can talk about "cross-cancelling." Notice that the 3 in the numerator came from the 3 of 8*3, while the 3 in the denominator came from the 9 of 9*5. In other words, the 3 in the numerator was from the numerator of the second fraction, 3/5, while the 3 in the denominator was from the denominator of the first fraction, 8/9. You don't need to write nearly as much as I did. You can just look at the problem 8 3 - * - = ? 9 5 and visualize it like this: 8*3 --- = ? 9*5 Then look for common factors in one number of the numerator and the other number of the denominator. We see that 3 is a factor of both 3 (in the numerator) and 9 (in the denominator). Divide each of these numbers by 3 (on paper, you'd cross out the 3 and write a 1 above it, and cross out the 9 and write a 3 below it): 8*1 --- = ? 3*5 You can look again for other common factors. Do the 8 and 5 have a common factor? No. Therefore we're done, and the product is 8*1 8 --- = -- 3*5 15 Do you see why it's called cross-cancellation? You look for common factors in these pairs: 8 3 \ / __ \/ __ /\ / \ 9 5 It's possible you might find common factors vertically, too - between the 8 and 9, or between the 3 and 5. But if you did, it would mean that the fractions you started with weren't in lowest terms. If the fractions are in lowest terms, then you only need to look for common factors in the "cross-terms." You can find more help in our Dr. Math Archives. Go to the Search Dr. Math page at http://mathforum.org/mathgrepform.html , and try searching for the words multiply fractions cross cancel - Doctor Rick, The Math Forum http://mathforum.org/dr.math/ |
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