WeBWorK using host: https://webwork-ptx.aimath.org, format: simple seed: 379

Does the center of a circle lie on the graph of the circle?

What is the radius of the circle $x^2 + y^2 = 81\text{?}$

What is the diameter of the circle in part (b)?

Can we simplify the equation in part (b) to $x + y = 9\text{?}$