JEE Main & Advanced JEE Main Paper (Held On 11 April 2015)

  • question_answer
    The solution of the differential equation \[ydx-(x+2{{y}^{2}})dy=0\]is\[x=f(y).\] If \[f(-1)=1,\] then\[f(1)\]is equal to: [JEE Main Online Paper (Held On 11 April 2015)]  

    A)  4

    B)  3

    C)  2

    D)  1

    Correct Answer: B

    Solution :

      \[y-\left( x+2{{y}^{2}} \right)\frac{dy}{dx}=0\]\[\Rightarrow \]\[y=\left( x+2{{y}^{2}} \right)\frac{dy}{dx}\] \[\Rightarrow \]\[y\frac{dy}{dx}=x+2{{y}^{2}}\]\[\Rightarrow \]\[\frac{dy}{dx}+\left( -\frac{1}{2} \right)x=2y\] \[I.F.={{e}^{\int_{{}}^{{}}{-\frac{1}{y}dy}}}={{e}^{-\ell ny}}={{y}^{-1}}={}^{1}/{}_{y}\] \[\therefore \]solution is \[x\left( \frac{1}{y} \right)=\int_{{}}^{{}}{\left( 2y \right)}\times \frac{1}{y}dy+c\] \[\Rightarrow \]\[\frac{x}{y}=2y+c\] \[x=1,y=-1\]\[\Rightarrow \]\[c=1\] \[\frac{x}{y}=2y+1\] put\[y=1\] \[x=2+1\] \[=3\]


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