20.设函数f(u)在 (0,+infty ) 内具有二阶导数,且 =f(sqrt ({x)^2+(y)^2}) 满足等式-|||-dfrac ({a)^2z}(a{x)^2}+dfrac ({a)^2z}(a{y)^2}=0.-|||-(1)验证 '(u)+dfrac (f'(u))(u)=0;-|||-(2)若 (1)=0, '(1)=1 求函数f(u)的表达式.