C2 Help please!

The circle with centre C (5,8) touches the y-axis,
express the equation of the circle in the form (x-a)^2+(y-a)^2=k
How do I find 'k' which is the radius?
Could you please explain?
Thank you!

Reply 1

martynsteel

11

If I remember correctly your 'k' value is the square root of your a value squared plus your b value squared :smile:

Reply 2

joostan

13

Original post by Monkeys!

The circle with centre C (5,8) touches the y-axis,
express the equation of the circle in the form (x-a)^2+(y-a)^2=k
How do I find 'k' which is the radius?
Could you please explain?
Thank you!

The equation of the circle should read that

\ (x-a)^2 + (y - b)^2 = r^2

What do you know already about a and b?

Reply 3

Munrot07

18

you know the center of the circle and you know that the circle touches the y axis when x = 0 so you can work out the coordinate at the y axis and then work out the distance between the center and the edge of the circle, giving you the radius, square the radius and you get k :smile:

obviously you know the center is at 5,8 and the place where it touches the y axis is 0,something so the distance is 5 so k is 25 (don't forget it is r^2 not r)

(edited 11 years ago)

Reply 4

Rumschpringe

11

Original post by Monkeys!

The circle with centre C (5,8) touches the y-axis,
express the equation of the circle in the form (x-a)^2+(y-a)^2=k
How do I find 'k' which is the radius?
Could you please explain?
Thank you!

If it touches the Y axis, and the x value of it's centre is 5, the radius must be 5, and therefore k=5 (diagram not to scale):

Reply 5

Ishraq Tarafdar

Original post by Munrot07

you know the center of the circle and you know that the circle touches the y axis when x = 0 so you can work out the coordinate at the y axis and then work out the distance between the center and the edge of the circle, giving you the radius, square the radius and you get k :smile:

You're a genius! I was stuck on that question myself haha.