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conic sections puzzler

I am stumped on a conic sections problem.
I am fouling up the algebra somehow, so I\\\'m not getting the same radius as the book I am working from. Here is the original problem:

x^2 + y^2 + 4y + 2x - 20 = 0

The book says I should get to the following step in the process of completing the square:

x^2 + y^2 - 4y + 2y - 4 = 0

But I just don\\\'t see where the -4y or the - 4 is coming from.

I get the following next step instead:

(x^2 + 2x + 1) + (y^2 - 4y + 4) = 20 + 1 + 4

This takes me to:

(x + 1)^2 + (y - 2)^2 = 25

then,

25 = r^2
so r = 5

But, I should be coming up with r = 3

Help!

746 day(s) ago

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