1.

Find the equation of the circle passing through the point (1, 4)and (5, 2) and has its centre on the line x - 2y + 9 =0.

Answer»

Let equation of the required circle is 

x2 + y2 > + 2gy + 2fy + c = 0

Given this equation passes through the points

(1,-4) and (5,2) 

(1,4) (1)2 + (-4)2 2g(1) + 2f(-4) + c = 0 

2g - 8f + C +17 = 0 ... (1) 

(5,2) 52 +22 + 2g (5) 2f(2) + c= 0 

10g + 4f + C + 29 = 0 .... (2) 

Also center (-g,-f) lies on the line x - 2 y + 9 = 0 

-g -2(-f) + c = 0 

-g + 2f + c = 0 ... (3) 

Solving equations 1,2 and 3 we get 

G = -3 f = 3,c = -47 

G = 1 f = 2, c = 4 

The required equation are 

x2 + y2 + 6x - y - 47 = 0 

x2 + y2 - 2x - 4y + 4 = 0



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