theorem Th7: :: PDIFF_3:7

for x0, y0 being Real
for z being Element of REAL 2
for f being PartFunc of (REAL 2),REAL st z = <*x0,y0*> & f is_hpartial_differentiable`21_in z holds
hpartdiff21 (f,z) = diff ((SVF1 (1,(pdiff1 (f,2)),z)),x0)