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.
Multiplying by 2 provides an equivalent statement to use as the first expression
The other thing we need to do is make m-n into n-m; we need to switch the order around. This basically means I want to make the positive m into a negative one and make the negative n into a positive n. Again, I can manipulate equations by doing the same thing to everything, so if I just multiply everything by -1 and distribute it out, my given information turns into -m+n equals -4. Because addition can be written in either order as long as the sign stays with the value, that's the same thing as n-m is equal to -4. We've found our second expression.
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Multiplying by -1 provides an equivalent statement to use as the second expression
Now that we've changed our given information into what we really want, all I have to do is substitute in the two new equations we came up with. So I now know that 2m-2n is 8 so I can substitute that in for the first part of my absolute value in the original question; I know that n-m is now -4 so I can now substitute that in. What I end up with is just |8| + |-4|.
Absolute values leave positive numbers alone so the 8 stays the same; they turn negative numbers positive so the -4 becomes positive, and 8+4 is 12.