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alexdok [17]
4 years ago
11

Determine whether the given differential equation is exact. if it is exact, solve it. (if it is not exact, enter not.) (3x + 6y)

dx + (6x − 8y3) dy = 0
Mathematics
1 answer:
ziro4ka [17]4 years ago
5 0
\underbrace{(3x+6y)}_{M(x,y)}\,\mathrm dx+\underbrace{(6x-8y^3)}_{N(x,y)}\,\mathrm dy=0

The ODE is exact if \dfrac{\partial M}{\partial y}=\dfrac{\partial N}{\partial x}. We have

\dfrac{\partial M}{\partial y}=6
\dfrac{\partial N}{\partial x}=6

so the equation is indeed exact.

We're looking for a solution of the form

\Psi(x,y)=C

By the chain rule, we have

\dfrac{\partial\Psi}{\partial x}+\dfrac{\partial\Psi}{\partial y}\dfrac{\mathrm dy}{\mathrm dx}=0
\iff\dfrac{\partial\Psi}{\partial x}\,\mathrm dx+\dfrac{\partial\Psi}{\partial y}\,\mathrm dy=0

which means \dfrac{\partial\Psi}{\partial x}=M and \dfrac{\partial\Psi}{\partial y}=N.

Now

\dfrac{\partial\Psi}{\partial x}=M\implies\displaystyle\int\frac{\partial\Psi}{\partial x}\,\mathrm dx=\int M\,\mathrm dx
\implies\Psi(x,y)=\dfrac32x^2+6xy+f(y)

Differentiating with respect to y yields

\dfrac{\partial\Psi}{\partial y}=N\implies 6x+f'(y)=6x-8y^3
\implies f'(y)=-8y^3\implies f(y)=\displaystyle\int(-8y^3)\,\mathrm dy=-2y^4+C

and so the general solution is

\Psi(x,y)=\dfrac32x^2+6xy-2y^4+C=C
\implies\Psi(x,y)=\dfrac32x^2+6xy-2y^4=C
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