Conics/Circles

[f1player]

Hi!

I'm having a little trouble with these sorts of questions:

1. Find the equation of the circle:

a) with centre (-1,-4) and radius 3

b) with centre (3,2) and passing through (5,-5)

For part 1 a) I did this:


a) (x-1)^2 + (y-4)^2 = 3^2

Is this right? Is that the equation of the circle?

For part b) i'm not sure what to do, can anyone please help?

thanks

 

[ting]

In the first one, the signs are incorrect.

(x+1)^2 + (y+4)^2 = 9



For the second substitute in what you know and solve for the radius.

 

[f1player]

I now understand part a)

However, I'm still not 100 % sure about part b)

Do i sub into this equation: (x-h)^2 + (y-k)^2 = r^2, where (h,k) is the centre of the circle?

If so, where do i sub in the (5,-5) value?

Any help would be appreciated

 

[ting]

You need to find r^2.

Substitute both the given centre (3,2) and the point (5, -5) into:


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

As you say the centre (3,2) is (h,k)

The point (5,-5) gives you your x and y values.

Evaluating the equation will give you r^2.

Then use the centre point and r^2 to write your equation as you did in the first question.

 

[f1player]

Yes, I think I understand now

For the equation I got: (x-3)^2 + (y-2)^2 = 53