1.

Determine the equation of the circle which touches the line y = x at the origin and bisects the circumference of the circle x2 + y2 + 2y − 3 = 0.

Answer»

Let S  x2 + y2 + 2gx + 2fy + c = 0 be the required circle. It passes through (0, 0). This implies c = 0. Now S = 0 touches the line x -  y = 0. Therefore

If S = 0 bisects the circumference of S'  ≡ x2 + y2 + 2y − 3 = 0, then S - S' = 0 passes through the centre (0, 1) of S' = 0. This implies that

2gx - 2(g + 1)y + 3 = 0

passes through (0, -1 )   (:. f = -g)

⇒ 0 -2(g + 1)(-1) + 3 = 0

⇒2g = -5 = -2f

Therefore, S ≡ x2 + y2  - 5x + 5y = 0



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