Question #304896

Test for exactness and find the complete solution of y/(x-1) dx+ {ln(2x-2)+ 1/y}dy=0

1
Expert's answer
2022-03-03T08:51:22-0500

We check the differential equation for exactness:


yx1dx+(ln(2x2)+1y)dy=0\dfrac{y}{x-1}dx+(\ln(2x-2)+\dfrac{1}{y})dy=0

M(x,y)=yx1,My=1x1M(x,y)=\dfrac{y}{x-1}, \dfrac{\partial M}{\partial y}=\dfrac{1}{x-1}

N(x,y)=ln(2x2)+1y,Nx=1x1N(x, y)=\ln(2x-2)+\dfrac{1}{y}, \dfrac{\partial N}{\partial x}=\dfrac{1}{x-1}

My=1x1=Nx\dfrac{\partial M}{\partial y}=\dfrac{1}{x-1}= \dfrac{\partial N}{\partial x}

We see that My=Nx,\dfrac{\partial M}{\partial y}= \dfrac{\partial N}{\partial x}, so that this equation is exact.


We have the following system of differential equations to find the function u(x,y)u(x, y)


ux=yx1\dfrac{\partial u}{\partial x}=\dfrac{y}{x-1}

uy=ln(2x2)+1y\dfrac{\partial u}{\partial y}=\ln(2x-2)+\dfrac{1}{y}

Hence


u(x,y)=yx1dx+φ(y)u(x, y)=\int\dfrac{y}{x-1}dx+\varphi(y)

=yln(x1)+φ(y)=y\ln(x-1)+\varphi(y)

Now, by differentiating this expression with respect to yy and equating it to uy,\dfrac{\partial u}{\partial y}, we find the derivative φ(y)\varphi'(y)


ln(x1)+φ(y)=ln(2x2)+1y\ln(x-1)+\varphi'(y)=\ln(2x-2)+\dfrac{1}{y}

φ(y)=ln2+1y\varphi'(y)=\ln2+\dfrac{1}{y}

φ(y)=(ln2+1y)dy=yln2+lnyC\varphi(y)=\int (\ln2+\dfrac{1}{y})dy=y\ln2+\ln y-C

Thus, the general solution of the differential equation is


yln(2x2)+lny=Cy\ln(2x-2)+\ln y=C

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