Question

Write the equation in standard form for the circle with radius 9 centered at the origin.

Answer 1

The standard form of the equation of a circle of radius r, with (assuming centre h, k) is given as:

(X-h)^2 + (y-k)^2 = r^2

As we are required to write an equation in standard form for the circle with radius 9 centred at the origin.
Centre(h,k)=(0,0), r=9

Substituting these values into the standard form of the equation of a circle given above:

(X-0)^2 + (y-0)^2 = 9^2
X^2 + y^2 =81

The standard form is x^2 + y^2 =81

I’m pretty sure this is right