How do I rotate a parabola from opening up to opening horizontally?

1 Answer
Aug 22, 2014

You can swap x and y to make it open to the right; this is the common one and is known as the inverse relation. Or you can swap -x with y to make it open to the left.

Let's do an example for the inverse relation of y=x^2+6x+8. We start by swapping x with y:

x=y^2+6y+8
x=(y^2+6y)+8 start completing the square for y
x=(y^2+6y+9)+8-9
x=(y+3)^2-1
x+1=(y+3)^2
+-sqrt(x+1)=y+3
-3+-sqrt(x+1)=y
y=-3+-sqrt(x+1)

You will want to notice that the inverse is a relation and not a function because it doesn't pass the vertical line test.

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