How to Evaluate Absolute Value Expressions

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  • 0:10 Solving by…
  • 1:51 Solving by Replacing…
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Lesson Transcript
Instructor: Zach Pino
Substituting values into absolute values doesn't have to be too hard, but it can be if you're given deceiving beginning information. See if you're up to it by checking out this video!

Solving by Substituting Variables

So we know that absolute values make things positive, but how does that effect how we evaluate absolute values at specific values? Let's take a look at an example:

Evaluate m|5-2n|+7, when m equals -2 and n equals 10. This problem is actually no more different than any other problem that asks us to substitute in values as long as we don't fall into the biggest absolute value pitfall; the absolute value bars DO NOT change subtraction symbols into addition ones. Yes, they do make things positive, but only after you've completed whatever operations are going on on the inside.

Treat the absolute value symbols like parenthesis and start solving inside them
Absolute Value Substituting Variables

So if we first substitute in -2 for m and 10 for n, we treat the absolute value bars like parentheses and we begin on the inside of them. Multiplication comes before subtraction, so I do 2*10 and I get 20. Then I do 5-20 and end up with -15, and only at that point, after I've finished all the different things on the inside of the absolute value, do we actually take the absolute value, making -15 positive 15. Again, because absolute value bars are kind of like parentheses, when you have a number in front it means multiplication, and -2*15 is -30. Last but not least, -30+7 is -23.

Solving by Replacing Groups of Variables

Now, where these problems can get kind of tricky is when the values of m and n aren't given to us quite so nicely. Take this example:

Evaluate |2m-2n| + |n-m| when m-n is equal to 4. So instead of simply telling us what n and m are individually, it only gives us what their difference is. It's going to be up to us to manipulate the given equation to make it look like what we want.

So what do we want? Well, the two expressions that would be nice to be able to substitute in are 2m-2n (that first absolute value) and n-m (that second one). If we knew both of those, we could simply plug in the numbers that we knew and be two steps away from our answer.

This means that it's up to us to learn what 2m-2n and n-m are from only the info we've got, which is m-n is 4.

So we need to turn m-n into 2m-2n. Luckily, we know that equations can be manipulated by doing the same thing to both sides to give us equivalent statements. So all I really need to do here is multiply everything in this equation by 2 because I want to turn m into 2m and I want to turn -1n into -2n. So multiplying the whole thing by 2 and distributing in gives us that 2m-2n is equal to 8. And we've already got one of our two expressions.

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