Question

(ii) Find the equation of the circle concentric with
the circle x² + y2 +10x +20y +20 = 0 and
passing through the point (1,-2).​

Answer 1

Given that we need to find the equation of the circle which is concentric with x2 + y2 - 3x + 4y - c = 0 and passing through (- 1, - 2).

We know that concentric circles will have same centre.

Let us assume the concentric circle be x2 + y2 - 3x + 4y + d = 0. ..... - (ii)

Substituting (- 1, - 2) in (ii) we get

⇒ (- 1)2 + (- 2)2 - 3(- 1) + 4(- 2) + d = 0

⇒ 1 + 4 + 3 - 8 + d = 0

⇒ d = 0

Substituting value of d in (ii) we get

⇒ x2 + y2 - 3x + 4y = 0