How would you find the center and radius of #x^2 + y^2 - 81 = 0#?

1 Answer
Dec 21, 2015

Answer:

Center : #(0,0)#; Radius : #9#.

Explanation:

First, you put the 81 at the right side, you're now dealing with #x^2 + y^2 = 81#.

You now recognize the square of the norm!

#x^2 + y^2 = 81 iff sqrt(x^2 + y^2) = sqrt81 = 9#.

It means that the distance between the origin and any point of the circle has to be equal to 9, you have to see #x^2# as #(x-0)^2# and #y^2# as #(y-0)^2# to see the origin appear. I hope I explained it well.