134.已知函数 y=y(x) 由方程 arctan dfrac (y)(x)=ln sqrt ({x)^2+(y)^2} 所确定,则 dfrac (dy)(dx)= ()-|||-A. dfrac (y-x)(y+x) B. dfrac (y+x)(y-x)-|||-C. dfrac (x-y)(x+y) D. dfrac (x+y)(x-y)