> 3 points make a circle, (-2,1) (2,1) and (-1,2). What is the radius of the circle?
Use the general equation of the circle: (x-a)^2 + (y-b)^2 = r^2, and
substitute in the co-ordinates for x and y.
The first two co-ordinates should lead you to see that (-2-a)^2 =
(2-a)^2, and expanding will give you an answer for a.
The third will give you b, and substituting these in the general
equation will give you r (which is (look away know if you don't want to
know) the square root of 5).
Without algebra, plotting the points should make the value of a obvious,
but I don't know if there is a more intuitive answer for the rest of the
equation.
Henry Logan
City of Bristol College, UK
hlogan@cityofbristol.ac.uk
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