Question

What is the radius of the circle given by the equation below?

x2 + 12x + y2 - 12y = -56
A. 2
B. 16
C. 4
D. 8

Answer 1

C

Explanation:

First, you gotta complete the square!

x^2+12x+36 = (x+6)^2

y^2-12y+36 = (y-6)^2

So, we have (x+12)^2 + (y-6)^2 = -56 + 36 + 36

The right side of the equation is the radius^2.

Simplify:

-56 + 36 + 36 = 16

The square root of 16 is 4, so the answer is C!