# Math Help - Equation of circle

1. ## Equation of circle

Find the equation of the circle which passes through (-1, 4)and with center at (4,-8)

I know this is an easy question but I just cant find the formula for how to find the equation. Any help appreciated.

2. I think you'd find the distance from the center to that point. That's the radius.

3. The standard form for the equation of a circle is:

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

Where the point $(h,k)$ is the center of the circle.

We are given that the center is found at (4,-8) so we have our equation so far:

$(x - 4)^2 + (y + 8)^2 = r^2$

Now we just need to find the radius. We have been given one point on the circle, so to find the radius we simply need to find the distance from the center of the circle to that point. Recall the distance formula:

$d = \sqrt {(x_2-x_1)^2+(y_2-y_1)^2}$

4. Ah. The answer is $(x-4)^2 + (y+8)^2 = 169$ from what you said about the distance from those 2 points.