[题目]设函数f w)具有二阶连续导数, =f((e)^xcos y)-|||-满足 dfrac ({partial )^2z}(partial {x)^2}+dfrac ({partial )^2z}(partial {y)^2}=(4z+(e)^xcos y)(e)^2x 若 (0)=0, '(0)=0, 求-|||-f(u)的表达式.