Circle Maths help. Watch

Guru Jason
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Got a example question in a lecture today and I don't understand it. Could someone help please?

I need to find the center and the radius of this circle.
x^2-4x+y^2+10y-7=0

so I was told that this was how it goes...
(x^2-4x+4)+(y^2+10y+25)=7+4+25

(x-2)^2+(y+5)^2=36

6^2=36

But I don't understand where the +4 and the +25 come from in the first line of working. Any help appreciated.
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TenOfThem
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(Original post by Guru Jason)
Got a example question in a lecture today and I don't understand it. Could someone help please?

I need to find the center and the radius of this circle.
x^2-4x+y^2+10y-7=0

so I was told that this was how it goes...
(x^2-4x+4)+(y^2+10y+25)=7+4+25

(x-2)^2+(y+5)^2=36

6^2=36

But I don't understand where the +4 and the +25 come from in the first line of working. Any help appreciated.

This is A Level, not undergraduate

But the bit you are asking about is Completing the Square which is GCSE knowledge

Do you know that

(x+a)^2 = x^2 + 2ax + a^2

and, therefore, that

x^2 + 2ax = (x+a)^2 - a^2
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Guru Jason
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(Original post by TenOfThem)
This is A Level, not undergraduate

But the bit you are asking about is Completing the Square which is GCSE knowledge

Do you know that

(x+a)^2 = x^2 + 2ax + a^2

and, therefore, that

x^2 + 2ax = (x+a)^2 - a^2
Thanks. It's a foundation year so I suppose it would be A level.
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TenOfThem
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(Original post by Guru Jason)
Thanks. It's a foundation year so I suppose it would be A level.
Ah - ok

So did you understand what I said or do you want me to go slower
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Guru Jason
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(Original post by TenOfThem)
Ah - ok

So did you understand what I said or do you want me to go slower
I think I got it now. I should be able to do the rest (the radius bit) of the questions now. Thanks.
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TenOfThem
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(Original post by Guru Jason)
I think I got it now. I should be able to do the rest (the radius bit) of the questions now. Thanks.
ok
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