Hello,
After a night,
f(x,y)=0 ==>df=0==>@f/@xdx+@f/@ydy=0
==>dy/dx=-(@f/@x)/(@f/@y) There was a mistake (in my memory)
@f/@x=6xy²+20x^4
@f/@y=5y^4+6x²y
==>dy/dx=-(6xy²+20x^3)/(5y^4+6x²y)
for x=-1 and y=2 dy/dx= -(6(-1)4+20(-1)^3)/(52^4+6(-1)²2)
=44/92=11/23
or
5y^4dy/dx+6xy²+6x²ydy/dx+20x^3=0
==>dy/dx(5y^4+6x²y)=-6xy²-20x^3
==>dy/dx= -(6xy²+20x^3)/(5y^4+6x²y)
==>11/23
