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# c2 circles watch

1. (Original post by lolman42)
subbing the equation of the line into the circle I get

7x^2 + 60x + 90 = 0

what do I do from here?
You must have realised that was wrong when you could not do part (a)
2. (Original post by Indeterminate)
Surely, as it's saying "write down", it's asking for

I think you have mis-read or mis-understood the question
3. (Original post by TenOfThem)
You must have realised that was wrong when you could not do part (a)
okay i am not too sure now

is the expansion 10x^2 + 60x + 100 = 10 ?

i did b^2-4ac but it does not equal
0

FORGET ALL WHAT i WROTE THERE!! i HAVE GOT IT NOW. what do I do for b now?
4. (Original post by lolman42)
okay i am not too sure now

is the expansion 10x^2 + 60x + 100 = 10 ?

i did b^2-4ac but it does not equal
0

FORGET ALL WHAT i WROTE THERE!! i HAVE GOT IT NOW. what do I do for b now?

So

You know know the point rom (a) so, sketch and use symmetry
5. (Original post by TenOfThem)
So

You know know the point rom (a) so, sketch and use symmetry
I still do not now the point

do I solve 10x^2 + 60x +90 =0 to find it?
6. (Original post by lolman42)
I still do not now the point

do I solve 10x^2 + 60x +90 =0 to find it?
yes

I suggest that you divide by 10 and use the fact that there is only one solution to spot the obvious factorisation
7. (Original post by TenOfThem)
yes

I suggest that you divide by 10 and use the fact that there is only one solution to spot the obvious factorisation
okay so x= -3
8. (Original post by lolman42)
okay so x= -3
Sketch and symmetry
9. TenOfThem's method is basically to find the intersection of the two curves, and show that there is only one intersection, so the line must be a tangent to the circle (think, if it crossed the circle there would have to be two solutions, one where it goes into the circle and one where it exits it).
10. (Original post by TenOfThem)
Sketch and symmetry
sorry, I am going to have to sound really silly here.

But I am lost
11. (Original post by lolman42)
sorry, I am going to have to sound really silly here.

But I am lost
Have you sketched?
12. (Original post by TenOfThem)
Have you sketched?
I am not exactly sure what I am meant to be sketching
13. (Original post by lolman42)
I am not exactly sure what I am meant to be sketching
A circle

and a line

14. (Original post by TenOfThem)
A circle

and a line

15. (Original post by lolman42)
So you have a circle

Then you have a line from (0,10) down to (-3,1)

As I say, symmetry will show you the other line
16. (Original post by TenOfThem)
So you have a circle

Then you have a line from (0,10) down to (-3,1)

As I say, symmetry will show you the other line
I can't see the symmetry..
17. (Original post by TenOfThem)
So you have a circle

Then you have a line from (0,10) down to (-3,1)

As I say, symmetry will show you the other line
surely there can only be one line passing through 0,10?

18. (Original post by lolman42)
surely there can only be one line passing through 0,10?
Well, infinite lines will pass through (0,10)

But there are 2 that pass through there and are tangents to the circle

I cannot work out why you cannot see it
19. (Original post by TenOfThem)

Well, infinite lines will pass through (0,10)

ohhh i was doing something different, so the coordinates at the new point of contact are 3,1?

But there are 2 that pass through there and are tangents to the circle

I cannot work out why you cannot see it
ohhh i was doing something different, so the coordinates at the new point of contact are 3,1?
20. (Original post by lolman42)
ohhh i was doing something different,
I am intrigued

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