dz/dx=2sin(x+y)*cos(x+y) dz/dy=2sin(x+y)*cos(x+y)
d(dz/dx)/dx=2cos(x+y)*cos(x+y)-2sin(x+y)*sin(x+y)
d(dz/dy)/dy=2cos(x+y)*cos(x+y)-2sin(x+y)*sin(x+y)
d(dz/dx)/dy=2cos(x+y)*cos(x+y)-2sin(x+y)*sin(x+y)
没错,三个一样...