Posted by **Anonymous** on Sunday, February 24, 2008 at 10:38am.

1) Circle centre (7,-2) passes through (-2,10). Calculate radius and find equation of circle.

2) AB is diameter of circle A(9,5) B(-1,-7). Find equation of circle mid point.

I know i have to use the eqn (x1+x2)/2 and (y1+y2)/2

3) Find the equation of circle with centre (-8,5). Which has y axis as tangent. Find equation of circle.

?? (x+8)²+(y-5)²=r² ??

Please help!

- maths -
**Damon**, Sunday, February 24, 2008 at 11:13am
1)

The radius is the distance from the center , (7,-2), to the point (-2,10)

I bet you know the distance formula:

r^2 = (X2 - X1)^2 + (Y2 - Y1)^2

Then

(x-7)^2 +(y+2)^2 = r^2

2)

Yes, the center is at the average of those two points on opposite sides of the circle.

I think it is also asking for the equation of the circle.

Once you have found the center, you can find the equation the same way you found it in #1 but for r^2 use the distance^2 from center to one of those given points.

3)

The y coordinate of the center is 5.

Therefore the tangent to the y axis will be 8 units to the right of the center and at y = 5. (Draw a sketch) In other words at (0,5)

The radius is therefore 8 and r^2 = 64

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