Verify the Pfaffian Differential equation
(y2+yz+z2)dx+(z2+zx+x2)dy+(x2+xy+y2)dz=0
is integrable and find its primitive.
The necessary and sufficient condition for iintegrability is
X⋅curlX=0, X=(y2+yz+z2,z2+zx+x2,x2+xy+y2)
So that
∇×X=∣∣i∂x∂y2+yz+z2j∂y∂z2+zx+x2k∂z∂x2+xy+y2∣∣
=i(x+2y−2z−x)−j(2x+y−y−2z)++k(z+2x−2y−z)=2(y−z)i+2(z−x)j+2(x−y)k
X⋅curlX=2(y2+yz+z2)(y−z)++2(z2+zx+x2)(z−x)+2(x2+xy+y2)(x−y)==2(y3−y2z+y2z−yz2+yz2−z3)++2(z3−xz2+xz2−x2z+x2z−x3)+=2(x3−x2y+x2y−xy2+xy2−y3)=0Thus the given equation is integrable.
P=y2+yz+z2,Q=z2+zx+x2,R=x2+xy+y2. The auxiliary equations are
∂z∂Q−∂y∂Rdx=∂x∂R−∂z∂Pdy=∂y∂P−∂x∂Qdz
2(z−y)dx=2(x−z)dy=2(y−x)dz
z−ydx=x−zdy=y−xdz
dx+dy+dz=0=>x+y+z=c1=u
(y+z)dx+(x+z)dy+(x+y)dz=0=>xy+yz+zx=c2=v Formulate the equation Adu+Bdv=0 and compare with the given equation.
Adx+Ady+Adz+Bxdy+Bydx+Bzdy+Bydz+Bzdx+Bxdz=0
A+By+Bz=y2+yz+z2A+Bx+Bz=z2+xz+x2A+By+Bx=x2+xy+y2
B=x+y+zA=−xy−yz−xz Then
A=−vB=u
Adu+Bdv=0−vdu+udv=0
vdv=udu
lnv=lnu+lncv=cu The required solution is
xy+yz+zx=c(x+y+z)
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