Question #100772
(y^2 +yz+ z^2)dx +(z^2 +zx+ x^2)dy + (x^2 +xy+ y^2)dz = 0
1
Expert's answer
2019-12-25T17:11:21-0500

Verify the Pfaffian Differential equation

(y2+yz+z2)dx+(z2+zx+x2)dy+(x2+xy+y2)dz=0(y^2+yz+z^2)dx+(z^2+zx+x^2)dy+(x^2 +xy+y^2)dz=0

is integrable and find its primitive.

The necessary and sufficient condition for iintegrability is


XcurlX=0,X\cdot curlX=0,

X=(y2+yz+z2,z2+zx+x2,x2+xy+y2)X=(y^2+yz+z^2,z^2+zx+x^2,x^2 +xy+y^2)

So that


×X=ijkxyzy2+yz+z2z2+zx+x2x2+xy+y2\nabla \times X=\begin{vmatrix} \mathbf i & \mathbf j & \mathbf k \\ {\partial \over \partial x} & {\partial \over \partial y} & {\partial \over \partial z} \\ y^2+yz+z^2 & z^2+zx+x^2 & x^2 +xy+y^2 \end{vmatrix}

=i(x+2y2zx)j(2x+yy2z)+=\mathbf i(x+2y-2z-x)- \mathbf j(2x+y-y-2z)++k(z+2x2yz)+\mathbf k(z+2x-2y-z)=2(yz)i+2(zx)j+2(xy)k=2(y-z)\mathbf i+2(z-x) \mathbf j+2(x-y) \mathbf k

XcurlX=2(y2+yz+z2)(yz)+X\cdot curlX=2(y^2+yz+z^2)(y-z)++2(z2+zx+x2)(zx)+2(x2+xy+y2)(xy)=+2(z^2+zx+x^2)(z-x)+2(x^2 +xy+y^2)(x-y)==2(y3y2z+y2zyz2+yz2z3)+=2(y^3-y^2z+y^2z-yz^2+yz^2-z^3)++2(z3xz2+xz2x2z+x2zx3)++2(z^3-xz^2+xz^2-x^2z+x^2z-x^3)+=2(x3x2y+x2yxy2+xy2y3)=0=2(x^3-x^2y+x^2y-xy^2+xy^2-y^3)=0

Thus the given equation is integrable.


P=y2+yz+z2,P=y^2+yz+z^2,Q=z2+zx+x2,Q=z^2+zx+x^2,R=x2+xy+y2.R=x^2 +xy+y^2.

The auxiliary equations are


dxQzRy=dyRxPz=dzPyQx{dx \over {\partial Q\over \partial z}-{\partial R \over \partial y}}={dy \over {\partial R\over \partial x}-{\partial P \over \partial z}}={dz \over {\partial P\over \partial y}-{\partial Q \over \partial x}}

dx2(zy)=dy2(xz)=dz2(yx){dx \over 2(z-y)}={dy \over 2(x-z)}={dz \over 2(y-x)}

dxzy=dyxz=dzyx{dx \over z-y}={dy \over x-z}={dz \over y-x}


dx+dy+dz=0=>x+y+z=c1=udx+dy+dz=0=>x+y+z=c_1=u

(y+z)dx+(x+z)dy+(x+y)dz=0=>xy+yz+zx=c2=v(y+z)dx+(x+z)dy+(x+y)dz=0=>xy+yz+zx=c_2=v

Formulate the equation Adu+Bdv=0Adu+Bdv=0 and compare with the given equation.

Adx+Ady+Adz+Bxdy+Bydx+Bzdy+Bydz+Bzdx+Bxdz=0Adx+Ady+Adz+Bxdy+Bydx+Bzdy+Bydz+Bzdx+Bxdz=0


A+By+Bz=y2+yz+z2A+By+Bz=y^2+yz+z^2A+Bx+Bz=z2+xz+x2A+Bx+Bz=z^2+xz+x^2A+By+Bx=x2+xy+y2A+By+Bx=x^2+xy+y^2

B=x+y+zB=x+y+zA=xyyzxzA=-xy-yz-xz

Then


A=vA=-vB=uB=u

Adu+Bdv=0Adu+Bdv=0vdu+udv=0-vdu+udv=0

dvv=duu{dv \over v}={du \over u}

lnv=lnu+lnc\ln v=\ln u+\ln cv=cuv=cu

The required solution is


xy+yz+zx=c(x+y+z)xy+yz+zx=c(x+y+z)

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