When adding and subtracting fractions, the important thing is to see if the fractions have the same denominator. Discover how to find the least common denominator with help from a math teacher in this free video on fractions in math.

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When adding and subtracting fractions, the important thing is to see if the fractions have the same denominator. Discover how to find the least common denominator with help from a math teacher in this free video on fractions in math.

Part of the Video Series: Fractions & Proportions

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So how do you add and subtract fractions? Hey, I'm Jimmy Chang. I've been teaching mathematics for nine years and you and I both know that fractions are not the best things to work with. But as you know they are indeed a necessary evil. And what we're going to go over today are ways that you can add and subtract fractions, like and unlike denominators. But that's the key. The importance is to see if the fractions that you're adding or subtracting are have the same denominator. So let's do an addition problem real quick. Suppose you want to find 3/12ths plus 2/12ths. One thing you want to be sure of is before you can add or subtract fractions is be sure that the denominators are the same. In this case they are. If the denominators are indeed the same then what you can do is you definitely keep the denominator as is always and then you just add across. Obviously 3 plus 2 is going to give you 5/12ths. So if you're adding or subtracting fractions with the same denominator then just add across the numerator and keep the denominator the same. Now what happens if they don't have the same denominator? Let's just say you have 3/8th minus 1/4th. Now obviously the denominators are not the same. So therefore you have to find one. And this is where the least common denominator comes from. Now the least common denominator is a denominator that both denominators will go into. In other words the denominators being 8 and 4 we have to ask ourselves what number will 8 and 4 go into? Well it just so happens of course that 8 will go into; 8 and 4 will go into the number 8. So being that that's the least common denominator you write the 8 for both fractions. And you're about to change them. Well because 8 and 8 are the same you keep the 3 as it is. But obviously you have to multiply 4 by something to get 8. Well 4 times what is 8? It has to be 2. And since; whenever you multiply the bottom you have to multiply the top by the exact same number. Negative 1 times 2 is negative 2. So 3/8th minus 1/4th is really 3/8ths minus 2/8ths. And so just like we did above you subtract across but you keep the denominator as is. So 3/8ths minus 1/4th is equal to 1/8th. So I'm Jimmy and these are some strategies on how to add and subtract fractions.