1.

Find the equation of the circle concentric with the circle x2 + y2 + 4x + 6y + 11 = 0 and passing through the point P(5, 4).

Answer»

Since circles are concentric, which means circles have common centre and different radii.

Equation of given circle, x2 + y2 + 4x + 6y + 11 = 0

The concentric circle will have the equation

x2 + y2 + 4x + 6y + d = 0 ….(1)

As it passes through P(5, 4),

Put x = 5 and y = 4

52 + 42 + 20 + 24 + d= 0

25 + 16 + 20 + 24 + d = 0

d = – 85

Equation (1) ⇒ x2 + y2 + 4x + 6y – 85 = 0

Which is required equation.



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