Question

The circle below is centered at the origin, and the length of its radius is 7. What is the circle's equation

A. 7x+7y=49
B. x^2+y^2=49
C. x^2+y^2=14

(Picture below.)

Answer 1

Equation of a circle is:
\[(x-h)^{2}+(y-k)^{2}=r ^{2} \]
h is the x-coordinate of the center and k is the y-coordinate of the center. Even though the formula has minus signs, those are positive numbers. If there were plus signs in there, the numbers h and k would actually be negative. Now since you're centered at the origin, that means h and k are 0, so you'd just have this:
\[x ^{2}+y ^{2}=r ^{2} \]
Now since your radius is 7, just square it and replace r^2 woth that value : )

Answer 2

Thank you!

Answer 3

Sure ^_^

Answer 4

Answer is B.

Answer 5

Bingo.