证明:函数-|||-f(x,y)= ((x)^2+(y)^2)sin dfrac (1)(sqrt {{x)^2+(y)^2}}, ^2+(y)^2neq 0,-|||-0, ^2+(y)^2=0-|||-在点(0,0)连续且偏导数存在,但偏导数在点(0,0)不连续,而f在点(0,0)可微.