1.

Find the equation of the circle which passes through origin, has its centre on the line x + y = 4 and cuts orthogonally the circle x2 + y2 − 4x + 2y + 4 = 0.

Answer»

Let S  x2 + y2 + 2gx + 2fy + c = 0 be the required circle. S = 0. The centre (−g, −f) lies on the line x + y = 4 implies that

-g - f = 4    ....(1)

S = 0 cuts the circle S'  x2 + y2 − 4x + 2y + 4 = 0 implies that

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

-4g + 2f = c + 4

-4g + 2f = 4  .......(2)

Solving Eqs. (1) and (2), we have g = −2 and f = −2. Therefore

 x2 + y2 - 4x - 4y = 0



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