Adding & Subtracting Fractions

Video Transcript

"Okay, now let's learn how to add and subtract fractions. So if we have a fraction three fourths and one fourth, now it's important that they have the same denominator in this example. What we do is we have the same denominator and just add the numerators. So the answer to this is three plus one is four and keep the same denominator, four; and that's equal to one. For subtracting fractions it works much from the same way, but it's also important here that we have the same denominator as well. Three quarters minus one quarter, you just take the first numerator, subtract the second numerator, three minus one is two, keep the same denominator again, two-fourths and that is equal to one half. When you're adding fractions with different denominators, there's another step that you need to do. If I add three-quarters and one third, I need to first make them have the common denominator. The easiest way to do that is to just multiply the two denominators together to get twelfths. So three quarters in twelfths I would have to do, so I'm going to change these to all have the same denominator. So to get twelve from four, I multiply by three, so I multiply the top by three as well. Three quarters is equivalent to nine twelfths. To get this denominator to be twelve, I multiply by four, so I must do the same for the numerator as well. Now that I have a fraction with the same denominator I can just add the numerators. That's how you add fractions with different denominators. Now to subtract fractions with different denominators, let's have another fraction say three quarters minus one third. You can't subtract them directly, so we need to change them to have a common denominator. What we're going to do here is we're going to make this into twelfths. To do that we multiply the top and bottom by three over three which is one and to get this into twelfths, we multiply the top and bottom by four. Three quarters is equivalent to nine twelfths and one third is equivalent to four twelfths. So we haven't changed the problem but we have gotten common denominators which will allow us to do the subtraction. Nine minus four is five and that's it, that's how you subtract fractions with different denominators."