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Question
Find the centre and radius of the circle
x2 + y2 – 22x – 4y + 25 = 0
Solution
Comparing x2 + y2 – 22x – 4y + 25 = 0 with general equation of circle x2 + y2 + 2gx + 2fy + c = 0
We get 2g = -22, 2f = -4, c = 25
g = -11, f = -2, c = 25
Centre = (-g, -f) = (11, 2)
Radius = `sqrt(g^2 + f^2 - c)`
`= sqrt((- 11)^2 + (-2)^2 - 25)`
`= sqrt(121+4-25)`
`= sqrt100` = 10
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