How do you find the center and radius of the circle #x^2 + y^2 - 8y = 9#?

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Answer 1 · 2018-03-18T12:23:07

#"centre "(0,4), " radius "=5#

we complete the square

#x^2+y^2-8y=9#

#rarrx^2+(y^2-8y+4^2)-4^2=9#

#x^2+(y-4)^2-16=9#

#x^2+(y-4)^4=25#

cmp with the standard eqn

#"centre "(a,b)," radius "=r

#(x-a)^2+(y-b)^"=r^2#

we have

#"centre "(0,4), " radius "=5#