Algebra 1 Variables
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X Marks the Spot
Variables

 

 What if I said:

 "I'm thinking of a number, and if you add 2 to it
 you get 5.
 What number am I thinking of?"
 
 Well that one's not too hard.
 We know that:
 

3 + 2 = 5

 
 So the number I was thinking of was 3.
 
 Math people are always asking questions like that
 (Hey, that's what they do!)
 So they wanted to come up with a quicker way
 of saying stuff like:
 "I'm thinking of a number. If you subtract 4 from it
 you get 2. What number am I thinking of?"
 
 At first, they got the bright idea to write:
 

? - 4 = 2

 

And then ask: What's the "?"

 

This particular "?" is 6, because 6 - 4 = 2

 
 Using a "?" for the number you were looking for
 worked fine for easy problems,
 but later harder problems they ran into troubles.
 So they decided that instead of a "?" they'd use an "X".
 
 So stuff like:

? - 4 = 2

 
 was now written as:

X - 4 = 2

 
 BUT IT STILL MEANS EXACTLY THE SAME THING.
 
 OK,
 I'm thinking of a number that if you add 2 to it, you get 6.
 What number am I thinking of?
 
 With a problem like this, 
 we can probably just look at it or count on our fingers
 and say:
 

If    X + 2 = 6   then X = 4

 

Because    4 + 2 = 6

 
 But as the problems get more complex,
 we start needing another way to get the answer.
 
 To deal with hard problems, 
 we have a set of rules we can use
 to find the thing we don't know (X).
 
 So start with a problem like:
 

X - 4 = 2

 
 and we want to wind up with:
 

X = (something)

 
 with the X all by itself,
 
 The first trick to help us do this says:
 

 
 So here, that means we add a 4 to each side
 to "get rid of" the - 4 next to the X
 

 
 Let's try a few more:
 
 Examples:
 

 

   copyright 2005 Bruce Kirkpatrick

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