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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. |
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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 S ≡ x2 + y2 - 4x - 4y = 0 |
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