A circle has equation x² + y² - 8x - 4y = 9
(i) Show that the centre of this circle is (4,2) and find the radius of the circle.
Circle equation = (x-a)² + (x-b)² = r²
The Attempt at a Solution
Well, if the centre of the circle is (4,2). Then the equation will be something like:
(x-4)² + (y-2)² = 9
except, if I expand out the two brackets, I get
(x² -8x + 16) + (y² -4y + 4) = 9
x² - 8x + y² -4y + 20 = 9
Here is where i get confused
So (i'm just double checking here since I don't have the answers), is the radius of the circle sqr9 or sqr 11? And is this really showing that (4,2) is the centre?