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Easy

Question

Let A be the vertex and L the length of the latus rectum of the parabola y2 – 2y – 4x – 7 = 0. The equation of the parabola with A as vertex, 2L the length of the latus rectum and the axis at right angles to that of the given curve is -

  1. x2 + 4x + 8y – 4 = 0    
  2. x2 + 4x + 8y – 12 = 0    
  3. x2 + 4x + 8y + 12 = 0    
  4. x2 + 8x – 4y + 8 = 0    

hintHint:

Let (y-k)2 = 4a(x-h) the (h,k) is the vertex and 4a is the length of latus rectum of the parabola and y = k is the axis of the parabola.

The correct answer is: x2 + 4x + 8y – 4 = 0


    Given, the equation y2 – 2y – 4x – 7 = 0
    y- 2y +1 = 4x + 7+ 1
    (y-1)2 = 4(x+2)
    here length of latus rectum L = 4 , vertex of parabola = (-2,1) and axis of parabola is  y = 1
    Then equation for perpendicular y = 1 and veterx (-2,1) is x = -2
    parabola length of latus rectum = 2L = 2*4 = 8
    the equation of parabola with axis x = -2 and latus rectum = 8 and vertex (-2,1) is
    left parenthesis x plus 2 right parenthesis squared space equals plus-or-minus 8 left parenthesis y minus 1 right parenthesis
x squared plus 4 x plus 4 space equals 8 y minus 8 space space space space space space space left parenthesis o r right parenthesis space x squared plus 4 x plus 4 space equals negative 8 y plus 8
x squared plus 4 x minus space 8 y plus 8 plus 4 space equals 0 space left parenthesis o r right parenthesis x squared plus 4 x plus space 8 y minus 8 plus 4 space equals 0
x squared plus 4 x minus space 8 y plus 12 space equals 0 space space space space space space left parenthesis o r right parenthesis space x squared plus 4 x plus space 8 y minus 4 equals 0
    Therefore, x2 + 4x + 8y – 4 = 0 is the parabola that with stand the coditions.

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