Year 12s: what will be the first way you research your future options? >>

C2 circle question.

The circle with equation x² - 6x+y² - 4y = 0 crosses the y-axis at the origin and the point A.

The first question asks me to find the co-ordinates of A. Now I feel this question is meant to be dead easy but I just can't see where to start with it.

I rearranged it to (x-3)² + (y-2)² = 13

But I still don't think that would help me.

Reply 1

Kevin De Bruyne

When something crosses the y axis, what does it tell you about one of the co-ordinates? Think of any co-ordinates across the y axis. They all have something in common.

Reply 2

I_Like_Maths

Original post by SeanFM

When something crosses the y axis, what does it tell you about one of the co-ordinates? Think of any co-ordinates across the y axis. They all have something in common.

That x = 0

Reply 3

Kevin De Bruyne

Bingo. Now find the y co-ordinate and you've found the co-ordinates of A. :cool:

Reply 4

I_Like_Maths

Original post by SeanFM

Bingo. Now find the y co-ordinate and you've found the co-ordinates of A. :cool:

Sorry for being really dumb but i'm still slightly confused. :s-smilie:

I'm confused am I not looking for a completely new point? If something passes through the origin then it's coordinates would be (0,0)

Sorry again for being really confused :rolleyes:

Reply 5

weasdown

You know that x=0 at A so stick zero in for x in the equation. That'll give you a nice quadratic in y.

Posted from TSR Mobile

Reply 6

Kevin De Bruyne

In some sense yes, you're looking for a new point. There are two points on the circle where x is 0. One of them is the origin as you've said, and the other one is also on the y axis but has a different y co-ordinate.

And above ^^

Oh, and some advice for circle questions, sketch them if you can. In this case a radius of sqrt 13 is a bit icky though. But if you called it 3.5 you'd probably be able to draw a helpful circle.