z=5-y
zx=0,zy=-1
原式=∫∫（x+y+5-y）√1+zx²+zy²dxdy
=∫∫（x+5）√2dxdy
=√2∫∫xdxdy+5√2∫∫dxdy
=0+5√2×π×5²
=125√2π