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# Coordinate geometry help - C2 Watch

1. (Original post by User32432432)
Sorry for the late reply, is this correct ?
(y+3)^2+9y
No you really must follow my advice and revise completing the square. Guessing like this is not helping your understanding.
2. (Original post by User32432432)
Can you just explain what I have done wrong ?
3. (Original post by Mr M)
No you really must follow my advice and revise completing the square. Guessing like this is not helping your understanding.
Oh, I understand now, its just in my notes I was working it out as if it was y^2+6y, but I only just realised its '-'
4. (Original post by User32432432)
Oh, I understand now, its just in my notes I was working it out as if it was y^2+6y, but I only just realised its '-'
That isn't your only problem with this. You are trying to add another term instead of subtract it and you have made this term a multiple of y when it isn't.
5. (Original post by Mr M)
That isn't your only problem with this. You are trying to add another term instead of subtract it and you have made this term a multiple of y when it isn't.
What do I do after this stage ? (y-3)^2 - 9
6. (Original post by User32432432)
What do I do after this stage ? (y-3)^2 - 9
Do the same with the x terms.

Note that the constant terms (i.e numbers) form the radius squared, namely
7. (Original post by User32432432)
What do I do after this stage ? (y-3)^2 - 9
Replace in the original equation and you have the equation of the circle in completed square form. This allows you to state the coordinates of the centre and the radius.

has centre (a, b) and radius r
8. (Original post by Indeterminate)
Do the same with the x terms.
There is nothing to do.
9. I think I know where I went wrong before when attempting to answer this (deleted it because it was wrong, I don't want anybody looking at it thinking it was right).
10. (Original post by Mr M)
There is nothing to do.
A bit foolish on my part to assume that he'd not dealt with the x terms already
11. (Original post by Mr M)
Replace in the original equation and you have the equation of the circle in completed square form. This allows you to state the coordinates of the centre and the radius.

has centre (a, b) and radius r
Thanks!
is r^2=16?
12. (Original post by User32432432)
Thanks!
is r^2=16?
Yes, well done
13. (Original post by raiden95)
Yes, well done
Finally lol

Thanks for your help, really appreciate it

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