13.(1)设函数 z=f(x,y) 由方程 (dfrac (y)(x),dfrac (z)(x))=0 确定,其中F为可微函数,且 _(2)=0,-|||-求 dfrac (partial z)(partial x)+ydfrac (partial z)(partial y)-|||-(2)设 =f((x)^2-(y)^2,(e)^xy), 其中f具有二阶连续偏导数,求 dfrac (partial z)(partial x) ,a2/Oy, dfrac ({e)^2z}(partial xpartial y).-|||-(3)设 =f(xy,dfrac (x)(y))+g(dfrac (y)(x)), 其中f具有二阶连续偏导数,g具有二阶连续导数,求-|||-dfrac ({partial )^2z}(partial xpartial y)-|||-(4)设函数 =f(x+y,x-y,xy), 其中f具有二阶连续偏导数,求dz与 dfrac ({partial )^2z}(partial xpartial y)-|||-,,,,