Answer:The equation of the circle is given as x^2 + y^2 = 81. In this equation, the coefficients of x^2 and y^2 are both 1, which implies that the circle is centered at the origin (0, 0).
To find the length of the circle's radius, we can use the formula:
r = sqrt(a^2 + b^2)
where a and b are the coefficients of x and y, respectively.
In this case, a = 1 and b = 1. Substituting these values into the formula, we get:
r = sqrt(1^2 + 1^2)
= sqrt(1 + 1)
= sqrt(2)
Therefore, the length of the circle's radius is sqrt(2) units.
Explanation: