TenMarks teaches you how to subtract fractions with unlike denominators.
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Learn about Subtracting Unlike Fractions In this lesson, let’s learn how to subtract fractions that are unlike. Unlike fractions means they have different denominators. So we’re going to use an example. The problem given to us is ¾ and from that I have to subtract ½. Different denominators, so they are unlike fractions. How do we do this? Well in order for us to do this, we have to first rename or convert one of the fractions so that both fractions have the same denominator. How do we do that? We did that during addition but let’s learn how to do it again. So I can take the fraction ½ because it’s got a smaller denominator and I want to get it so that the denominator is 4 which is the same as the other one. So in order to do that, I can multiply the top and bottom by 2. 2×2 is 4, 1×2 is 2. Remember that we have to multiply the top and bottom by the same number. The numerator and denominator have to be multiplied by the same number. Well that gives me these two are equivalent fractions. They are worth the same. Let me show you using an example. I have a sheet of paper. I have broken it up into two halves, ½ and ½. If I break the same sized paper into four quarters, what do I have? 1 quarter, 1 quarter, 1 quarter and 1 quarter. As you can see, half is the same as 2 quarters. They are exactly the same. Of course my sketch is not exactly like what it is but you get the idea. ½ is the same as 2/4. This is the piece. Since that’s the same, I can say ½ is equivalent to 2/4. So now what do we have? Instead of subtracting these, I can subtract ¾ and from that, instead of ½ I can put 2/4 because these are the same. If two denominators are the same, we copy them and we subtract the numerator. 3-2 is1, that’s the answer we’re looking for. Subtract the numerator to get the numerator, and copy the denominator. What do we get? ¼. Now the key thing to remember is if they both have the same denominators, only then can we subtract. So first, if they have different denominators, we have to convert one of them or both of them so they have the same denominator.

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