z=（sinx）^3+y^3-3xy^2
dz/dx=3(sinx)^2*cosx-3y^2
dz/dy=3y^2-6xy
d^2z/dx^2=d(dz/dx)/dx=d[3(sinx)^2*cosx-3y^2]/dx=6sinx*(cosx)^2-3(sinx)^3
d^2z/dx^2=d(dz/dy)/dy=d(3y^2-6xy)/dx=6y-6x
d^2z/dxdy=d(dz/dy)/dx=d(dz/dx)/dy=-6y