z = f(e^xsiny，x-2y)∂z/∂x = e^xsinyf'<1> + f'<2>∂^z/∂x∂y = e^xcosyf'<1> + e^xsiny[e^xcosyf''<11>-2f''<12>]+ [e^xcosyf''<21>-2f''<22>]= e^xcosyf'<1>+ (1/2)e^(2x)sin2yf''<11>+e^x[cosy-2siny)f''<12> - 2f''<22>
