7.求下列函数的 dfrac ({sigma )^2z}(partial {x)^2} . dfrac ({a)^2x}(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.