Question #105513
a) Verify that the total differential equation
yz dx +(x^2y-zx)dy+(x^2z-xy)dz=0 is integrable and hence find its integral.
1
Expert's answer
2020-04-03T18:19:50-0400

The given differential equation is ,

yzdx+(x2yzx)dy+(x2zxy)dz=0yzdx+(x^2y-zx)dy+(x^2z-xy)dz=0

We known that ,the necessary and sufficient condition of integrability of the given differential equation Pdx+Qdy+Rdz=0Pdx+Qdy+Rdz=0 is



P(QzRy)+Q(RxPz)+R(PyQx)=0P(\frac{\partial Q}{\partial z}-\frac{\partial R}{\partial y})+Q(\frac{\partial R}{\partial x}-\frac{\partial P}{\partial z} )+R(\frac{\partial P}{\partial y}-\frac{\partial Q}{\partial x})=0

...........(1)...........(1)

Now ,according to question,

P=yz,Q=(x2yzx) and R=(x2zxy)P=yz,Q=(x^2y-zx) \ and \ R=(x^2z-xy)

Now ,

Py=z and Pz=y.\frac{\partial P}{\partial y}=z \ and \ \frac{\partial P}{\partial z}=y.Qx=(2xyz) and Qz=x\frac{\partial Q}{\partial x}=(2xy-z) \ and \ \frac{\partial Q}{\partial z}=-xRx=(2xzy) and Ry=x\frac{\partial R}{\partial x}=(2xz-y) \ and \ \frac{\partial R }{\partial y}=-x



putting above all value in equation (1) ,we get ,

yz(x+x)+(x2yzx){(2xzy)(y)}+(x2zxy){(z)(2xyz)}yz(-x+x)+(x^2y-zx)\{(2xz-y)-(y) \}+(x^2z-xy) \{(z)-(2xy-z)\}

=0+2(x3yzx2y2x2z2+xyz+x2z2x3yzxyz+x2y2)=0=0+2(x^3yz-x^2y^2-x^2z^2+xyz+x^2z^2-x^3yz-xyz+x^2y^2)=0

Hence ,given differential equation is integrable.

To solve the given differential equation , we take

x=constant.      dx=0x=constant. \ \implies \ dx=0 .

 (x2yzx)dy+(x2zxy)dz=0\therefore \ (x^2y-zx) dy+(x^2z-xy)dz=0

     x2(ydy+zdz)x(zdy+ydz)=0\implies \ x^2(ydy+zdz)-x(zdy+ydz)=0

     x2(ydy+zdz)xd(yz)=0\implies \ x^2(ydy+zdz)-xd(yz)=0

On integrating ,we get


x2y22+x2z22xyz=ϕ ........(2)\frac{x^2 y^2} {2}+\frac{x^2z^2}{2}-xyz=\phi \ ........(2)

Where ϕ\phi is a constant, it may be regarded as a function of x.x.

Differentiating (2) w.r.t. x , y, and z we get,

(xy2dx+yx2dy)+(xz2dx+zx2dz)(yzdx+xzdy+xydz)=dϕ(xy^2dx+yx^2dy)+(xz^2dx+zx^2dz)-(yzdx+xzdy+xydz)=d\phi

On simplifying, we get

(yx2xz)dy+(zx2xy)dz+yzdx+(xy2+xz22yz)dx=dϕ ...........(3)(yx^2-xz)dy+(zx^2-xy)dz+yzdx+(xy^2+xz^2-2yz)dx=d\phi \ ...........(3)

Since ,yzdx+(yx2xz)dy+(zx2xy)dz=0 (given equation)yzdx+(yx^2-xz)dy+(zx^2-xy)dz=0 \ (given \ equation) ,therefore equation (3) is,

(xy2+xz22yz)dx=dϕ ........(4)(xy^2+xz^2-2yz)dx=d\phi \ ........(4)

Again from equation (2),


x2y22+x2z22xyz=ϕ\frac{x^2y^2}{2}+\frac{x^2z^2}{2}-xyz=\phi

     x2y2+x2z22xyz=2ϕ\implies \ x^2y^2+x^2z^2-2xyz=2\phi

Now dividing both side by x ,we get


     xy2+xz22yz=2ϕx............(5)\implies \ xy^2+xz^2-2yz=\frac{2\phi}{x}............(5)

From equation (4) and (5) ,we get



2ϕxdx=dϕ\frac{2\phi }{x} dx=d\phi     2dxx=dϕϕ\implies \ \frac{2dx}{x}=\frac{d\phi }{\phi}

On integrating ,we get

logx2+logk=logϕlogx^2+logk=log\phi

Where k is a constant.

     log(x2k)=logϕ\implies \ log(x^2k)=log\phi

     x2k=ϕ\implies \ x^2k=\phi

Now ,putting the value of ϕ\phi in equation (2) ,we get


x2y22+x2z22xyz=x2k\frac{x^2y^2}{2}+\frac{x^2z^2}{2}-xyz=x^2k

     xy2+xz22yz=2xk\implies \ xy^2+xz^2-2yz=2xk






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