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!
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!