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Question

Find the circumcentre of the triangle whose vertices are (-2, -3), (-1, 0), (7, -6).

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Solution

Let the circumcentre be O(x,y)
Circumcentre is equidistant from the vertices.
OA= OB = OC

OA = square root of left parenthesis x plus 2 right parenthesis squared plus left parenthesis y plus 3 right parenthesis squared end root
OB = (x+1)2+y2
OC = square root of left parenthesis x minus 7 right parenthesis squared plus left parenthesis y plus 6 right parenthesis squared end root

OA = OB
OAblank squared=OBblank squared
left parenthesis x plus 2 right parenthesis squared plus left parenthesis y plus 3 right parenthesis squared equals left parenthesis x plus 1 right parenthesis squared plus y squared x squared plus 4 plus 4 x plus y squared plus 9 plus 6 y equals x squared plus 1 plus 2 x plus y squared 13 plus 4 x plus 6 y equals 2 x plus 1 2 x plus 6 y plus 12 equals 0 x plus 3 y plus 6 equals 0
x + 3y + 6 = 0 ----- (i)

OB=OC
OBblank squared=OCblank squared
left parenthesis x plus 1 right parenthesis squared plus y squared = left parenthesis x minus 7 right parenthesis squared plus left parenthesis y plus 6 right parenthesis squared
x squared plus 1 plus 2 x plus y squared equals x squared plus 49 minus 14 x plus y squared plus 36 plus 12 y 2 x plus 1 equals 85 minus 14 x plus 12 y 16 x minus 12 y minus 84 equals 0 4 x minus 3 y minus 21 equals 0

4x-3y-21 =0 ----- (ii)

Add i and ii
5x-15=0
x=3
y=-3
Circumcentre is (3,-3)


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