13.(单选题,3.0分)-|||-曲面 z=f(x,y) 在点(x0,y0,z0)处的切平面法向量与Z轴夹角为锐角,则该法向量方向余弦中的cosy等于 ()-|||-A A dfrac (1)(sqrt {{{f)_(x)}^2((x)_(0),(y)_(0))+({f)_(y)}^2((x)_(0),(y)_(0))+1}}-|||-B 1-|||-C -1-|||-D dfrac (-1)(sqrt {{{f)_(x)}^2((x)_(0),(y)_(0))+(f)_({x)^2}((x)_(0)(y)_(0))+1}} √h