# obtaining an equation from the equation of a circle (given a point)

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• June 21st 2012, 03:29 PM
kingsolomonsgrave
obtaining an equation from the equation of a circle (given a point)
Answer is to be in the form Ax + By + C = 0

the pines passes through (3,-5) and through the center of the circle

(x+1)^2 + (y-3)^2 = 25/4
• June 21st 2012, 03:44 PM
Reckoner
Re: obtaining an equation from the equation of a circle (given a point)
Quote:

Originally Posted by kingsolomonsgrave
Answer is to be in the form Ax + By + C = 0

the pines passes through (3,-5) and through the center of the circle

(x+1)^2 + (y-3)^2 = 25/4

You should easily be able to find the center of the circle from that equation. If not, look up the standard form of the equation of a circle.

Can you find the line that passes through the two points? Can you at least find the slope?
• June 24th 2012, 09:41 AM
kingsolomonsgrave
Re: obtaining an equation from the equation of a circle (given a point)
the center of the circle is (-1, 3) and the slope is -2 I think so the equation would be y = -2x+1

is that right?
• June 24th 2012, 09:47 AM
Reckoner
Re: obtaining an equation from the equation of a circle (given a point)
Quote:

Originally Posted by kingsolomonsgrave
the center of the circle is (-1, 3) and the slope is -2 I think so the equation would be y = -2x+1

That's correct. But the question said to put the answer in $Ax + By + C = 0$ form, so get everything on one side.
• June 24th 2012, 09:54 AM
kingsolomonsgrave
Re: obtaining an equation from the equation of a circle (given a point)
so 2x+y-1=0
• June 24th 2012, 09:55 AM
Reckoner
Re: obtaining an equation from the equation of a circle (given a point)
Quote:

Originally Posted by kingsolomonsgrave
so 2x+y-1=0

Right.
• June 24th 2012, 09:59 AM
kingsolomonsgrave
Re: obtaining an equation from the equation of a circle (given a point)
thanks!