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Let \mathrm{y=y(x)} be the solution of the differential equation \mathrm{\left(3 y^2-5 x^2\right) y \mathrm{~d} x+2 x\left(x^2-y^2\right) \mathrm{d} y=0}such that \mathrm{y(1)=1}. Then \mathrm{\left|(y(2))^3-12 y(2)\right|} is equal to :
 

Option: 1

16 \sqrt{2}
 


Option: 2

32 \sqrt{2}
 


Option: 3

32


Option: 4

64


Answers (1)

best_answer

\mathrm{ \left(3 y^2-5 x^2\right) y d x+2 x\left(x^2-y^2\right) d y=0 }
\mathrm{ 2 x\left(x^2-y^2\right) d y=\left(5 x^2-3 y^2\right) y d x }
\mathrm{\Rightarrow \frac{d y}{d x}=\frac{\left(5 x^2-3 y^2\right) y}{2 x\left(x^2-y^2\right)}}
Let \mathrm{\mathrm{y}=\mathrm{tx}}

\mathrm{\frac{d y}{d x}=t+x \frac{d t}{d x}}
\mathrm{ t+x \frac{d t}{d x}=\frac{\left(5-3 t^2\right) t}{2\left(1-t^2\right)} }
\mathrm{x \frac{d t}{d x}=t\left[\frac{5-3 t^2-2+2 t^2}{2\left(1-t^2\right)}\right]}
\mathrm{ =\frac{t}{2}\left[\frac{3-t^2}{1-t^2}\right] }
\mathrm{ \frac{1}{3} \int \frac{3\left(1-t^2\right) d t}{3 t-t^3}=\int \frac{d x}{2 x} }
\mathrm{\frac{1}{3} \ln \left|3 t-t^3\right|=\frac{1}{2} \ln |x|+c }
\mathrm{2 \ell \ln \left|3 t-t^3\right|=3 \ln |x|+6 c }
\mathrm{ \left(3 t-t^3\right)^2=x^3 \lambda }
\mathrm{ \left(\frac{3 y}{x}-\frac{y^3}{x^3}\right)^2=x^3 \lambda }
\mathrm{ \left(3 y x^2-y^3\right)^2=x^9 \lambda }
\mathrm{ x=1 \Rightarrow y=1}
(3-1)^2=1 \times \lambda
\lambda=4
\mathrm{\left(3 y x^2-y^3\right)^2=4 x^9}
Let \mathrm{x=2}
\mathrm{ \left(3 y(2) \times 4-\left(y(2)^3\right)^2\right)=4(2)^9 }
Taking square root both the sides,

\mathrm{ \left|(\mathrm{y}(2))^3-12 \mathrm{y}(2)\right|=2(2)^{9 / 2} }

\mathrm{ =2(2)^4 \sqrt{2}=32 \sqrt{2}}

 

Posted by

vishal kumar

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