Question 299600
The equation of the inner circle is x2 + y2 = 9. The radius of the outer circle is three times the radius of the inner circle. What is the equation of the outer circle

The standard form of a circle with center at (h,k) and radius r is:

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

The inner circle's equation can be rewritten as:

(x-0)^2 + (y-0)^2 = 3^2 and so has center at (0,0) and radius 3.

Since the outer circle also has center at (0,0) but has radius 3*3 = 9 its equation is:

(x-0)^2 + (y-0)^2 = 9^2 or

x^2 + y^2 = 81