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# Help with Circle question, A level C2.

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1. Hi all,

I have been re-visiting old topics recently in prep. for the c2 exam. Today I am doing circles, but I can't do this question. I know I have done it before, but I have forgotten which makes it even more frustrating

Show that the circle with equation x² + y² - 2ax - 2bx + b² = 0 touches the Y-axis.

Thanks a lot for any help
2. At the y-axis you have x=0

Set x=0 and see what happens
3. (Original post by TenOfThem)
At the y-axis you have x=0

Set x=0 and see what happens
Hi, I did that and got y² + b² = 0

But how does this show that it touches the y axis?

Thanks
4. I think that you have copied the question incorrectly and you should have -2by not -2bx
5. I don't think you can answer this question unless you have the values for a and b as they're the numbers that give you the translation from your original circle..
6. (Original post by TenOfThem)
I think that you have copied the question incorrectly and you should have -2by not -2bx
I double checked and the way I copied it is the way its in the book, but maybe this is a typo in the book? Out of interest, what would you do if it was -2by?
Attached Thumbnails

7. If it is -2by

When x=0, y=b ... a single solution so the circle touches the y-axis at (0,b)

So, yes, I think it is a typo in the book
8. (Original post by TenOfThem)
If it is -2by

When x=0, y=b ... a single solution so the circle touches the y-axis at (0,b)

So, yes, I think it is a typo in the book
Thats makes sense, thanks a lot!

Stupid textbooks =_=

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