# pre-calculus

posted by .

Determine the center and the radius of the following circle:

x^2+y^2+8x-2y=8

so I figured I'd use completing the square after putting it in standard form:

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

I'm not sure where to go from here.

• pre-calculus -

but you didn't "complete" the square

(x^2+8x)+(y^2-2y)=8
(x^2+8x + 16)+(y^2-2y + 1)= 8 + 16 + 1
(x+4)^2 + (y-1)^2 = 25

Now we have it in standard form.
Can you take it from there ?

• pre-calculus -

Ok. Do I take the square root of the whole thing now?

getting x+4-y-1+-sqrt25
x-y=3+-sqrt25

I've never done this before, I'm not sure if this is even right and if it is, I definitely don't know where to go from here

• pre-calculus -

no,
the standard equation of a circle with centre (h,k) and radius r is
(x-h)^2 + (y-k)^2 = r^2

so we ended up with
(x+4)^2 + (y-1)^2 = 25 or
(x+4)^2 + (y-1)^2 = 5^2

then the centre is (-4,1) and radius is 5

Here is an interesting page for your topic
http://www.analyzemath.com/CircleEq/Tutorials.html

• pre-calculus -

Thanks so much for your explanation. I was able to figure it out and now got your answer.

## Similar Questions

1. ### Algebra

Write the standard form of the equation of a circle that passes through the points at (0,8) (8,0) and (16,8). Then identify the center and radius of the circle. (x-8)^2 + (y-8)^2 = 64 (x^2 / 64) + (y^2 / 64) = 64 center=(8,8) radius=8 …
3. ### Math

The points (4, –5) and (– 4, 1) are endpoints of a diameter of a circle. (a) Find the center of the circle. (b) Find the length of the radius of the circle. (Note that this is a distance.) Give the exact answer. Show work. (c) …
4. ### Pre-Calc

Determine the center and the radius of the following circle: x^2+y^2+6x-4y-5=0 I got for the radius 3(sqrt2) I got for the center (-3,2) Can you please tell me if I am correct?
5. ### Calculus

Complete the square to write the equation x^2 + y^2 - 6x - 2y + 4 = 0 in standard form. Use the standard form of the equation to identify the center and the radius of the circle.

I can get part of the answer then I get stumped. find the center and radius of the circle, Write the standard form of the equation. (4,3)(1,3) to get center: (1+4/2, 3+3/2)=5/2,3 answer:5/2,3 for center radius: then i am stuck
7. ### pre calculus

Write the equation of the circle in standard form. Find the center, radius, intercepts, and graph the circle. x^2+y^2-12x-4x+36=0 How on earth do I do this?
8. ### college pre-calculus

4x^2+4y^2-16x-24y+51=0 is the equation of a circle. where is the center of the circle and what is it's radius?
9. ### Geometry

Write the General form equations for the following circles: 1. A circle with center (1, -6) and radius 4 2. A circle with center (6, 8) and radius 10 3. A circle with center (0, 3) and radius 2√3 4. A circle with center (-0.5, 5.5 …
10. ### Pre-calculus

Find the center radius and equation of a circle in standard form given the following conditions: 1. Tangent to 3x+2y=0 at the point (0,0) and passing through (1,-1) and (6,0)

More Similar Questions

Post a New Question