How to Use Addition to Check Subtraction

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Though it may seem counter intuitive, one way to check subtraction problems is through the use of addition. Use addition to check subtraction with help from a longtime mathematics educator in this free video clip.

Part of the Video Series: Principles of Math
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Hi, I'm Jimmy Chang, and we're here to talk about how to use addition to check subtraction. Now, addition and subtraction are very closely related in that they're opposite operations. So, to use addition to check subtraction can be a pretty straightforward process. Here are a couple of examples. You know, for example, that nine minus three is going to equal to six. Now, obviously, this is subtraction. But, to use addition to check it, what you can do is you can use a little bit of algebra to kind of help you there, and that's where the addition part comes in. So, to verify and to check that nine minus three is equal to six, what you can do is you can add three to both sides. That's where using the addition comes from. And you have nine is equal to six plus three. So, if nine minus three equals six is indeed true, then to check it then by adding three to both sides, then you can say that six plus three is equal to nine. So, that's a way of checking things. Now, another way to do this would be if you have, let's just say one hundred minus twenty is equal to eighty, since you know that's true. Again, you can verify it by checking it by adding the number that's being subtracted on both sides. So, if you add twenty to both sides, you have one hundred is equal to eighty plus twenty. That's going to give you a hundred. Or, you can think of it this way: Leave the two numbers as they are. One-hundred is equal to eighty plus twenty, and you have it checked for you. Because, if it's true that one hundred minus twenty is equal to eighty, then it also has to be true that one hundred is equal to eighty plus twenty. Basically, what you're doing is you're taking the answer of the subtraction problem, and you're adding the number that was being subtracted from the original, and you're pretty much done. So, I'm Jimmy Chang, and that's how to use addition to check subtraction.

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