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Find the vertex, focus, axis, directrix and lotus – rectum of the following parabolas(i) y2 = 8x(ii) 4x2 + y = 0(iii) y2 – 4y – 3x + 1 = 0(iv) y2 – 4y + 4x = 0(v) y2 + 4x + 4y – 3 = 0 |
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Answer» (i) y2 = 8x Given: Parabola = y2 = 8x Now by comparing with the actual parabola y2 = 4ax Then, 4a = 8 a = 8/4 = 2 So, the vertex is (0, 0) The focus is (a, 0) = (2, 0) The equation of the axis is y = 0. The equation of the directrix is x = – a i.e., x = – 2 The length of the latus rectum is 4a = 8. (ii) 4x2 + y = 0 Given: Parabola => 4x2 + y = 0 Now by comparing with the actual parabola y2 = 4ax Then, 4a = 1/4 a = 1/(4 × 4) = 1/16 So, the vertex is (0, 0) The focus is = (0, -1/16) The equation of the axis is x = 0. The equation of the directrix is y = 1/16 The length of the latus rectum is 4a = 1/4 (iii) y2 – 4y – 3x + 1 = 0 Given: Parabola y2 – 4y – 3x + 1 = 0 y2 – 4y = 3x – 1 y2 – 4y + 4 = 3x + 3 (y – 2)2 = 3(x + 1) Now by comparing with the actual parabola y2 = 4ax Then, 4b = 3 b = 3/4 So, the vertex is (-1, 2) The focus is = (3/4 – 1, 2) = (-1/4, 2) The equation of the axis is y – 2 = 0. The equation of the directrix is (x – c) = -b (x – (-1)) = -3/4 x = -1 – 3/4 = -7/4 The length of the latus rectum is 4b = 3 (iv) y2 – 4y + 4x = 0 Given: Parabola y2 – 4y + 4x = 0 y2 – 4y = – 4x y2 – 4y + 4 = – 4x + 4 (y – 2)2 = – 4(x – 1) Now by comparing with the actual parabola y2 = 4ax => (y – a)2 = – 4b(x – c) Then, 4b = 4 b = 1 So, the vertex is (c, a) = (1, 2) The focus is (b + c, a) = (1-1, 2) = (0, 2) The equation of the axis is y – a = 0 i.e., y – 2 = 0 The equation of the directrix is x – c = b x – 1 = 1 x = 1 + 1 = 2 Length of latus rectum is 4b = 4 (v) y2 + 4x + 4y – 3 = 0 Given: The parabola y2 + 4x + 4y – 3 = 0 y2 + 4y = – 4x + 3 y2 + 4y + 4 = – 4x + 7 (y + 2)2 = – 4(x – 7/4) Now by comparing with the actual parabola y2 = 4ax => (y – a)2 = – 4b(x – c) Then, 4b = 4 b = 4/4 = 1 So, The vertex is (c, a) = (7/4, -2) The focus is (- b + c, a) = (-1 + 7/4, -2) = (3/4, -2) The equation of the axis is y – a = 0 i.e., y + 2 = 0 The equation of the directrix is x – c = b x – 7/4 = 1 x = 1 + 7/4 = 11/4 Length of latus rectum is 4b = 4. |
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