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# circle geometry help watch

1. Hey there
I am really lost with the question and would appreciate help

I just don't know how to go about it and the mark scheme is not helping at all

thanks
Hey there
I am really lost with the question and would appreciate help

I just don't know how to go about it and the mark scheme is not helping at all

thanks
Complete the square on the LHS, and note that circles have equations of the form

where r is the radius, and (a,b) is the centre.

Hint:

3. (Original post by Indeterminate)
Complete the square on the LHS, and note that circles have equations of the form

where r is the radius, and (a,b) is the centre.

Hint:

Okay i complete the square to get :

(x-8)^2 + (y + a/2)^2

therefore

45= 64 - (a/2)^2

And it's not giving me the correct answer
Okay i complete the square to get :

(x-8)^2 + (y + a/2)^2

therefore

45= 64 - (a/2)^2

And it's not giving me the correct answer
Look at the final line.

You've subtracted them both on the LHS, so they have to be positive on the RHS with the -20

45= 64 - (a/2)^2
You forgot about the 20 and you have a sign error.
6. (Original post by Indeterminate)
Look at the final line.

You've subtracted them both on the LHS, so they have to be positive on the RHS with the -20
it is the 20 part I am confused about on the mark scheme.. why do we include it? thanks
it is the 20 part I am confused about on the mark scheme.. why do we include it? thanks
You have to include it because that's part of the "broken down" form of the equation that was given to you on the paper.
it is the 20 part I am confused about on the mark scheme.. why do we include it? thanks
When you say "why do we include it" it would be more valid to enquire why you excluded it?
9. (Original post by Indeterminate)
You have to include it because that's part of the "broken down" form of the equation that was given to you on the paper.
what exactly is the 20? the radius? a coordinate? a constant?
what exactly is the 20? the radius? a coordinate? a constant?
A constant. You don't do anything to it when you complete the square, so why exclude it?
Okay i complete the square to get :

(x-8)^2 + (y + a/2)^2

therefore

45= 64 - (a/2)^2

And it's not giving me the correct answer
So

So

So

And you are told that the RHS = 45
12. (Original post by Indeterminate)
A constant. You don't do anything to it when you complete the square, so why exclude it?
I am really confused still... surely we only need the centre coordinates and the radius to find a? ( i know now i am wrong.. but how?)
I am really confused still... surely we only need the centre coordinates and the radius to find a? ( i know now i am wrong.. but how?)
That is what you need

Well, actually, as I have shown, you only need the radius
I am really confused still... surely we only need the centre coordinates and the radius to find a? ( i know now i am wrong.. but how?)
If I have

then

by the definition of a circle.

15. Note 'R' and rearrange!

Posted from TSR Mobile
16. (Original post by TenOfThem)
So

So

So

And you are told that the RHS = 45
Thanks TenOfthem

So where do i go from here?
Thanks TenOfthem

So where do i go from here?
The RHS = 45

I am sure that you can solve

Thanks TenOfthem

So where do i go from here?
You know so solve for 'a' and show you can get the required solutions
19. (Original post by TenOfThem)
The RHS = 45

I am sure that you can solve

hmmm haha

yea I hopefully will be able too

thanks tenofthem btw, nominated you for most helpful member of the month

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