Q17 / 105 · NTA Abhyas (JEE Main) 2020 · Mathematics · Differential Equations

If the general solution of the differential equation y'=yx+ϕxy, for some function ϕ , is given by yln|cx|=x, where c is an arbitrary constant, then ϕ2 is equal to here, y'=dydx

  • A
    -4
  • Correct answer
    B
    - 1 4
  • C
    1 4
  • D
    4

Solution

given  :  y ′ = y x + ϕ x y

 As     y ln cx = x  ⇒  y ′ ln cx + y 1 cx c = 1

                     ⇒  y ′ x y + y x = 1

⇒    1 - y x x y = y x + ϕ x y

x = 2 ,  y = 1  ⇒  1 - 1 2 2 1 = 1 2 + ϕ 2 1

⇒    ϕ 2 = - 1 4

Hence, the correct answer is option: 'B'.

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