Addition Equations with One-Digit Integers

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  • 0:02 Addition Equations
  • 0:35 Subtraction Property…
  • 2:23 Example 1
  • 2:51 Example 2
  • 3:29 Lesson Summary
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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

After watching this video lesson, you will be able to solve equations where you need to find a missing number. Learn the steps you need to take to find a missing number and how subtraction can help you.

Addition Equations

Imagine that you and your friend were just given a math worksheet to finish. This math worksheet is titled 'Addition Equations'. It includes two integers added together and equaling another integer. The interesting thing about these addition equations, though, is that they have a variable for one of the integers. It is your job to find out what this missing integer is. Your problems look like these:

8 + x = 9

x + 5 = 7

The Subtraction Property of Equality

How do you go about finding what this missing integer, this variable x, is equal to? You can use the subtraction property of equality, which says that if you subtract the same thing from both sides of an equation, your equation will still be the same. What happens when you subtract? Let me show you.

Take the first problem, for example. 8 + x = 9. If we subtract 8 from the left side, what are we left with? 8 + x - 8 = ? We are left with x. That's what we want. We want our variable x by itself on one side of the equation so that we'll know what it equals. The subtraction property of equality tells us that we now also need to subtract the 8 from the right side. We have 9 - 8 = 1. What does our x equal? Replacing both sides with what they now equal, we have 8 + x - 8 = 9 - 8, becoming x = 1.

Look at that! That's a nice equation telling us exactly what x equals. It equals 1. And we're done! We can use this property to help us solve any kind of addition equations where we have a variable plus an integer equaling another integer. To find our variable, we subtract the integer that is being added to our variable from both sides of the equation. What we're left with is our answer.

We can check our answer, too, by plugging our answer back into the original problem to see if it works. For our first problem, we have 8 + 1 = 9. Is this a true statement? Does 8 + 1 really equal 9? It does. We get 9 = 9, which we know is true.

Let's look at a couple more examples.

Example 1

x + 5 = 7

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