1.

Find the equation of the circle passing through the point (0,5) and (6, 1) and has its centre on the line 12x + 5y = 25.

Answer»

Let the equation of the required circle is 

x2 + y2 + 2gx + 2fy + c = 0 given that this 

Equation passes through the points (0,5) and (6, 1) 

∴ (0,5) 02 + 52 + 2(0)g + 2(5)f + C=0 

10f + C + 25 = 0 

(6, 1) 62 + 12 + 2.6g + 2.1f+C = 0 …. (1) 

12g + 2f + C + 37 = 0 …. (2) 

Also the centre (-g, -f) lies on the line 

12x + 5y – 25 = 0 

∴ -128 – 5f – 22 = 0 …. (3) 

Equation (2) – Equation (1) gives 

12g – 8f + 12 = 0 …(4) 

Adding equations 3 and 4 we get 

-13f – 13 = 0 → f = \(\frac{13}{-13}\) - 1

⇒ f = -1 

From equation 4 

12g = 8f + 12 = – 8 + 12 = 4 

g = \(\frac{1}{3}\)

10 + C + 25 = 0 ⇒ c = -15 

∴ Required equation is x2 + y2 + 2\(\frac{1}{3}\)x + 2(-1)y -15 = 0

⇒ 3x2 + 3y2 + 2x – y – 45 = 0



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