7.求下列函数的 dfrac ({partial )^2z}(partial {x)^2} dfrac ({partial )^2z}(partial {y)^2} 和 dfrac ({partial )^2z}(partial xpartial y) =-|||-(1) =(x)^4+(y)^4-4(x)^2(y)^2 ;-|||-(2) =arctan dfrac (y)(x) :-|||-(3) =(y)^x 。