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# c2 circles help (new kid) Watch

1. hi there ppl

i have question!

show that the circle x^2 + y^2 - 2ax - 2bx + b^2 = 0 touches y-axis

thnax
2. any 1?
3. (Original post by lolman42)
hi there ppl

i have question!

show that the circle x^2 + y^2 - 2ax - 2bx + b^2 = 0 touches y-axis

thnax

Assuming that it's 2by ,you need to show that the line x=0 is a tangent to the circle.

So set x equal to zero and observe the nature of the resulting quadratic in y.

Hint:

for a tangent,

4. (Original post by Indeterminate)
Assuming that it's 2by ,you need to show that the line x=0 is a tangent to the circle.

So set x equal to zero and observe the nature of the resulting quadratic in y.

Hint:

for a tangent,

but the resulting quadratic has a b^2 term and a y^2 term after setting the x's to zero in the circle equation
5. (Original post by Indeterminate)
Assuming that it's 2by ,you need to show that the line x=0 is a tangent to the circle.

So set x equal to zero and observe the nature of the resulting quadratic in y.

Hint:

for a tangent,

I am unsure still :/
6. (Original post by lolman42)
but the resulting quadratic has a b^2 term and a y^2 term after setting the x's to zero in the circle equation
Are you sure you've not made a mistake in copying out the Q?

Otherwise, when we're on the y-axis

which is impossible.
7. (Original post by Indeterminate)
Are you sure you've not made a mistake in copying out the Q?

Otherwise, when we're on the y-axis

which is impossible.
yep this is it: x^2 + y^2 - 2ax - 2bx + b^2 = 0

I have triple checked :P
8. (Original post by Indeterminate)
Are you sure you've not made a mistake in copying out the Q?

Otherwise, when we're on the y-axis

which is impossible.
what shall I do? :/
9. (Original post by lolman42)
yep this is it: x^2 + y^2 - 2ax - 2bx + b^2 = 0

I have triple checked :P
so there isn't a 'y' term but there are 2 'x' terms: -2ax and -2bx?
10. (Original post by davros)
so there isn't a 'y' term but there are 2 'x' terms: -2ax and -2bx?
Yeap- I have checked meticulously

It's exactly how it is in my book
11. (Original post by lolman42)
Yeap- I have checked meticulously

It's exactly how it is in my book
I think the question must be wrong then... as pointed out earlier, when x=0 you have y^2 + b^2 = 0 which is impossible!
12. (Original post by davros)
I think the question must be wrong then... as pointed out earlier, when x=0 you have y^2 + b^2 = 0 which is impossible!
Yeh, I thought that.. thanks bud
13. (Original post by lolman42)
hi there ppl

i have question!

show that the circle x^2 + y^2 - 2ax - 2bx + b^2 = 0 touches y-axis

thnax
Certainly an error in your book

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