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#1 2006-03-08 05:18:40

RickyOswaldIOW
Member
Registered: 2005-11-18
Posts: 212

Equation of a Circle.

This circle has a center point at (1, 2) and a radius of 3.
According to the genral equation of a circle, that's:
(x - 1)² + (y - 2)² = 3²

expanded:
x² - 2x + 1 + y² - 4y + 4 = 9

and
x² - 2x + y² - 4y = 4?

my book is saying (y - 2) (y - 2) = y² - 2√2y!
(y - 2) (y - 2) = y² - 2y - 2y + 4 in my eyes?!


Aloha Nui means Goodbye.

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#2 2006-03-08 12:50:57

Ansette
Member
Registered: 2006-02-19
Posts: 21

Re: Equation of a Circle.

must be a mistake in the book...

the equation u have is indeed correct..

im curious as to why ur looking at (y-2)²...?

the x²-2x can be simplified to x(x-2)

so:

x(x-2) +y²-4y=4

what it seems you're doing is saying (y-2)² = y²-4y+4.. which is true..

however what you have in this equation is:

x(x-2) = -y²+4y+4

which using the quadratic formula forms:

y=1±√2

so you have x(x-2) = (y-[1+√2])(y-[1-√2])

or aternatively:

x(x-2) = (y-[1+√2])(y+[√2-1])

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#3 2006-03-09 05:14:23

RickyOswaldIOW
Member
Registered: 2005-11-18
Posts: 212

Re: Equation of a Circle.

im curious as to why ur looking at (y-2)²...?

My question asks "Give the equation of a circle with center (1, 2), radius 3"
and the genral equation for a circle is "(x - a)² + (y - b)² = r²"
hence (y - 2)².

From that question, the book states that the answer is;
(x - 1)² + (y - √2)² = 3
expanded
x² - 2x + y² - 2√2y = 0

very bizarre!


Aloha Nui means Goodbye.

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#4 2006-03-09 05:28:52

Ansette
Member
Registered: 2006-02-19
Posts: 21

Re: Equation of a Circle.

if this is alevel im presuming it's edexcel.. there are a lot of mistakes in the answers bit lol

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#5 2006-03-10 05:14:36

RauLiTo
Member
Registered: 2006-01-11
Posts: 142

Re: Equation of a Circle.

that is our lesson big_smile cool
well explain your question more and u will get the answer wink


ImPo$$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

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#6 2006-03-11 00:34:10

amonjezi
Member
Registered: 2006-03-09
Posts: 9

Re: Equation of a Circle.

must be mistakes in your books
what is your name book

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#7 2006-03-13 10:49:00

RickyOswaldIOW
Member
Registered: 2005-11-18
Posts: 212

Re: Equation of a Circle.

It is indeed edexcel A-level.  The book is Core 2 and I study through the National Extensions College.


Aloha Nui means Goodbye.

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