34.设函数f(x,y)有连续二阶偏导数.满足 dfrac ({partial )^2f}(partial xpartial y)=0, 且在极坐标系下可表成 f(x,y)=-|||-g(r),其中 =sqrt ({x)^2+(y)^2}, 求f(x,y).C1(a^ )+c2