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

1 Answer
Topscooter
Dec 21, 2015

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.

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