Teacher2Teacher Q&A #18923

Teachers' Lounge Discussion: Subtracting fractions

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From: Loyd

To: Teacher2Teacher Public Discussion
Date: 2010122116:58:55
Subject: Subtraction fractions

I liked Ralph's explanation. My experience in teaching algebra students is that this type problem is one that causes problems. I have heard other teachers also say that the teaching of algebra is hampered because many students are weak in fractions. So, I will give another example a little harder. Subtract 5 and 2/3 from 10 and 3/7. Rewrite: 10 3 --- 7 5 2 --- 3 First thing is to get the least common denominator (LCD) which is 21. So we multiply the first fraction by 3/3 and the second fraction by 7/7. The new problems is" 10 9 --- 21 5 14 ---- 21 ----------- Now we borrow a "whole" as Ralph's said and change it to 21/21 and add that to 9/21 and then we can subtract as usual. The new problem is: 9 30 --- 21 5 14 ---- 21 Now they are "like" fractions so we can subtract. ----------- 4 16 --- 21 The trick here is that we can always multiply by 1 and not change the value. 3/3 and 7/7 were used as multipliers and both of them are equal to 1. Make sure students can are proficient in finding the LCD and LCM even if denominators are not prime.

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