Kalabamboo
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Just stuck on iv
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oakjpw
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The portion of the line between A and B will be a diameter. The center is therefore the midpoint. Hope that starts you off!
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Kalabamboo
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(Original post by oakjpw)
The portion of the line between A and B will be a diameter. The center is therefore the midpoint. Hope that starts you off!
But the portion of the line between A and B could possibly be a chord.
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mqb2766
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The perpendicular bisector is the set of points equidistant from A & B. Find where this intersects with the x=3 line and that does the job?
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oakjpw
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(Original post by Kalabamboo)
But the portion of the line between A and B could possibly be a chord.
It can't be a chord because you are told the centre lies on that portion of the line. Any line joining two points of the circumference through the centre is a diameter.
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Kalabamboo
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(Original post by mqb2766)
The perpendicular bisector is the set of points equidistant from A & B. Find where this intersects with the x=3 line and that does the job?
Why specifically the intersection of the perpendicular bisector line with the line x=3, why not the intersection of the line AB with the line x=3?
The mark scheme has the same method as you I think -just dont understand this part
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3pointonefour
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(Original post by Kalabamboo)
Just stuck on iv
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Your equation is (x-3)^2 + (y-b)^2 = r^2
The two points a and b lie on the circle, so they satisfy the equation.
Create 2 simultaneous equations by plugging in the x and y coordinates for A and B and solve simultaneously.
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oakjpw
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That is overcomplicating a lot for just 4 marks I think.
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Kalabamboo
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(Original post by oakjpw)
It can't be a chord because you are told the centre lies on that portion of the line. Any line joining two points of the circumference through the centre is a diameter.
"A circle passing through A and B has its centre on the line x = 3. Find the centre of the circle
and hence find the radius and equation of the circle"
The question doesn't say that the centre lies on the AB line. It just says that the circle passes through the points A and B and that the circle has its centre on the line x=3
Here is the mark scheme
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Kalabamboo
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(Original post by mqb2766)
The perpendicular bisector is the set of points equidistant from A & B. Find where this intersects with the x=3 line and that does the job?
Why specifically the intersection of the perpendicular bisector line with the line x=3, why not the intersection of the line AB with the line x=3?
The mark scheme has the same method as you I think -just dont understand this part
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3pointonefour
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(Original post by Kalabamboo)
"A circle passing through A and B has its centre on the line x = 3. Find the centre of the circle
and hence find the radius and equation of the circle"
The question doesn't say that the centre lies on the AB line. It just says that the circle passes through the points A and B and that the circle has its centre on the line x=3
Here is the mark scheme
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This is just a bit of loci. The perpendicular bisector have you a line of points which are the same distance from a and b. So the point where x = 3 has the same distance from a and b, much like a radius. So using the perpendicular bisector works here since it's not necessarily a diameter.
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mqb2766
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(Original post by Kalabamboo)
Why specifically the intersection of the perpendicular bisector line with the line x=3, why not the intersection of the line AB with the line x=3?
The mark scheme has the same method as you I think -just dont understand this part
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My first reaction was to use quadratics and find r^2 for the center lying at (3.y) and then solve for y, but this is too much for 4 marks.

Just draw the picture of the line AB and the perpendicular bisector. All the points on the perpendicular bisectior must be the same distance from A & B, so the circle's center must lie on this line. If you're told the center also lies on x=3, then you simply substitute x=3 into the equation for the perpendicular bisector (intersection of the two lines).

Just draw the lines and remember you can construct the perpendicular bisector using a pair of compasses and drawing circles centered at A & B. Once you understand it graphically, it's fairly obvious. AB is a chord, the center must lie on the perpendicular bisector, something like https://www.bbc.com/education/guides/zsv8xfr/revision/7
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Kalabamboo
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(Original post by mqb2766)
My first reaction was to use quadratics and find r^2 for the center lying at (3.y) and then solve for y, but this is too much for 4 marks.

Just draw the picture of the line AB and the perpendicular bisector. All the points on the perpendicular bisectior must be the same distance from A & B, so the circle's center must lie on this line. If you're told the center also lies on x=3, then you simply substitute x=3 into the equation for the perpendicular bisector (intersection of the two lines).

Just draw the lines and remember you can construct the perpendicular bisector using a pair of compasses and drawing circles centered at A & B. Once you understand it graphically, it's fairly obvious. AB is a chord, the center must lie on the perpendicular bisector, something like https://www.bbc.com/education/guides/zsv8xfr/revision/7
Thanks a lot
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