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write a rule

how do I write a rule for:
 
    x   -1, 0, 1, 2
    y    1, 3, 9, 27
 
 
I think it is x add 1, and y multiply by 3
 
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  x   -1, 0, 1, 2
  y    3, 1, 1/3, 1/9
 
How do I write a rule for this function?
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1 Answer

I think you mean equation, expressing y in terms of x. But that's just terminology. Anyway, you're pretty close, because you figured out they multiply by 3. That means that y is some power of three.
 
In this case, when x=0, y=3. So 3=3^(0+1). let's check out work.
(-1,1) : 1=3^(-1+1). Check.
(1,9) :  9 = 3^(1+1). Yes.
(2,27) : 27=3^(2+1). Yes!
 
So: y=3^(x+1). Be sure to check all the numbers to be sure!