12.求下列函数的 dfrac ({a)^2z}(partial {x)^2} dfrac ({a)^2z}(partial xpartial y) ,(a^2z)/(av^2) (其中f具有二阶连续偏导数):-|||-(1) =f(xy,y);-|||-(2) =f(x,dfrac (x)(y));-|||-(3) =f(x(y)^2,(x)^2y);-|||-(4) =f(sin x,cos y,(e)^x+y).