The equation of a circle is typically written in the form of (x – h)² + (y – k)² = r², where (h, k) is the center of the circle and r is the radius.
In the equation provided, x² + y² = 9, we can see that it closely resembles the standard form. Here, the center of the circle is at (0, 0) because there are no linear x or y terms. To identify the radius, we compare this equation to the standard form:
r² = 9
To find the radius (r), we take the square root of both sides:
r = √9 = 3
Thus, the radius of the circle whose equation is x² + y² = 9 is 3.