What is the differential equation of the family of parabolas having a vertex at origin and axis along positive y-axis?

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NDA 02/2022 Mathematics Official Paper (Held On 04 Sep 2022)
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  1. x + 2y = 0
  2. x − 2y = 0
  3. y + 2x = 0
  4. y − 2x = 0

Answer (Detailed Solution Below)

Option 2 : x − 2y = 0
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NDA 01/2025: English Subject Test
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Detailed Solution

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Concept:

The equation of the parabola having a vertex at the (h,k) and axis along the positive y-axis is (x - h)2 = 4a(y - k)

Calculation:

The equation of the family of parabolas having vertex at origin and axis along the positive y-axis

⇒ (x - 0)2 = 4a(y - 0) 

⇒ x2 = 4ay  __(i)

Differentiating both sides with respect to x,

⇒ 2x = 4a

⇒ x = 2a  __(ii)

Putting the value of a from (i) in (ii),

⇒ x = 2 

⇒ 2xy = x2

⇒ x − 2y = 0

 

∴ The correct option is (2).

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