Let M=phi(IR^2) be a surface such that phi(u,v)=(u+v , u-v, uv). Z is the unit vector space of the M such as f(p)=p+rZ(p) and f:M->Msubr is a bijective function.For the Msubr surface find the following;
a)Gauss and mean curvature at the point of f(5,-1,6)
b)Determine the umbilical points