1.

If x + y = 3 is the equation of the chord AB of the circle x2 + y2 – 2x + 4y – 8 = 0, find the equation of the circle having bar AB as diameter.

Answer»

Required equation of circle passing through intersection S = 0 

and L = 0 is S + λL = 0 

(x2 + y2 – 2x + 4y – 8) + λ(x + y – 3) = 0 

(x2 + y2 + x(–2 + λ) + y (4 + λ) – 8 – 3λ = 0 —— (i) 

x2 + y2 + 2gx + 2fy + c = 0 —— (ii) 

Comparing (i) and (ii) we get

g = (-2 + λ)/2, f = (4 + λ)/2

Centre lies on x + y = 3

∴ - ((-2 + λ)/2) - ((4 + λ)/2) = 3

2 – λ – 4 – λ = 6 

–2λ = 8 ⇒ λ = – 4 

Required equation of circle be 

(x2 + y2 – 2x + 4y – 8) – 4(x + y – 3) = 0 

x2 + y2 – 6x + 4 = 0



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