1.

Find the equation of the circle passing through the points (4, 1) and (6, 5) and whose center is on the line 4x + y = 16.

Answer»

Let the equation of the circle be

x2 + y2 + 2gx + 2fy + c = 0 ______(1)

Since (1) passes through (4, 1)

16 + 1 + 8g + 2f + c = 0

⇒ 8g + 2f + c = -17 ______(2)

Since (1) passes through (6, 5)

36 + 25 + 12g + 10f + c = 0

⇒ 12 g + 10f + c = -61 _______(3)

(1) – (2) ⇒ -4g – 8f = 16

⇒ -g – 2f = 4 ______(4)

Since center is on the line 4x + y = 16, we have;

⇒ -4g – f = 16 ______(5)

Solving (4) and (5) We get; g = -3; f = -4

(2) ⇒ 8(-3) + 2(-4) + c = -17

⇒ -24 – 8 + c = -17 ⇒ c = 15

Then the equation of the circle is

x2 + y2 – 6x – 8y + 15 = 0.



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