u=f(x²-y²，e^xy)那么求偏导数得到u'x=f1' *2x+f2 '*e^xy *y再对y求偏导数此时x看作常数u''xy=-4xy*f11''+2x² *e^xy *f12''+f2' *(1+xy)*e^xy -f21''*2e^xy *y²+f22'' *e^2xy *xy
