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Show that the points (3, -2), (1, 0), (-1, -2) and (1, -4) are concyclic. |
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Answer» Let the equation of the circle passing through the points (3, -2), (1, 0) and (-1, -2) be x2 + y2 + 2gx + 2fy + c = 0 …..(i) For point (3, -2), Substituting x = 3 and y = -2 in (i), we get 9 + 4 + 6g – 4f + c = 0 ⇒ 6g – 4f + c = -13 ….(ii) For point (1, 0), Substituting x = 1 and y = 0 in (i), we get 1 + 0 + 2g + 0 + c = 0 ⇒ 2g + c = -1 ……(iii) For point (-1, -2), Substituting x = -1 and y = -2, we get 1 + 4 – 2g – 4f + c = 0 ⇒ 2g + 4f – c = 5 …….(iv) Adding (ii) and (iv), we get 8g = -8 ⇒ g = -1 Substituting g = -1 in (iii), we get -2 + c = -1 ⇒ c = 1 Substituting g = -1 and c = 1 in (iv), we get -2 + 4f – 1 = 5 ⇒ 4f = 8 ⇒ f = 2 Substituting g = -1, f = 2 and c = 1 in (i), we get x2 + y2 – 2x + 4y + 1 = 0 ……….(v) If (1, -4) satisfies equation (v), the four points are concyclic. Substituting x = 1, y = -4 in L.H.S of (v), we get L.H.S. = (1)2 + (-4)2 – 2(1) + 4(-4) + 1 = 1 + 16 – 2 – 16 + 1 = 0 = R.H.S. Point (1, -4) satisfies equation (v). ∴ The given points are concyclic. |
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