11.设 z=f(x,y) 二次可微,且 =(e)^ucos v =(e)^usin v, 试证:-|||-dfrac ({partial )^2z}(partial {x)^2}+dfrac ({partial )^2z}(partial {y)^2}=(e)^-2u(dfrac ({partial )^2z}(partial {u)^2}+dfrac ({partial )^2z}(partial {y)^2})