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If the equation of parabola is y2 = -12x, then which of the following is true ?
1. Equation of axis is: y = 0
2. Equation of directrix is: x - 3 = 0
3. Equation of latus rectum is: x + 3 = 0
4. All of the above
5. None of these

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Correct Answer - Option 4 : All of the above

Concept:

For a parabola of the form y2 = - 4ax where a > 0 we have:

  • Focus is given by: (- a, 0)
  • Vertex is given by: (0, 0)
  • Equation of directrix is given by: x - a = 0
  • Equation of axis is given by: y = 0
  • Equation of latus rectum is given by: x + a = 0
  • Length of latus rectum is given by: 4a


Calculation:

Given: Equation of parabola is :  y2 = - 12x

We can re-write the equation of given parabola as:

⇒ y2 = - 4 ⋅ (3) ⋅ x

Now by comparing the above equation with y2 = - 4ax where a > 0 we get: a = 3.

So, equation of directrix is: x - 3 = 0

Similarly, equation of axis is: y = 0 and equation of latus rectum is: x + 3 = 0.

Hence, option D is true.

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