Question

Write the equation of the circle whose center is (-4,8) and has a diameter of 18

Answer 1

Answer:

(x + 4)^2 + (y - 8)^2 = 81

or

(x + 4)^2 + (y - 8)^2 = 9^2 depending on how your teacher wants it written.

Step-by-step explanation:

The standard form for a circle is

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

r is the radius.

You are given the diameter

r = d/2

r = 18/2

r = 9

So you already have the right hand side of the equation

(x + h)^2 + (y + k)^2 = 9*2

(x + h)^2 + (y + k)^2 = 81

You basically have h and k as well. They come from the center point.

h = 4

k = - 8

So the equation of the circle (and the answer) is

(x + 4)^2 + (y - 8)^2 = 81

One question remains. Why do the x and y values change signs? If you know what the distance formula is, then what you are finding is the distance r of all points on the circle from the center of the circle.

It is the distance formula that is actually the formula for the circle.