B
对方程x+cos(x+y)=0两边取微分,得 dx - sin(x+y)d(x+y)=0
即 dx - sin(x+y)dx+sin(x+y)dy=0,整理得[1- sin(x+y)]dx= - sin(x+y0dy
从而 ｜dy/dx｜=| [1- sin(x+y)]/sin(x+y) | （*）
当x=0时,代入原方程 得y=pai/2,
再把求得的y=pai/2,x=0代入（*）式得 ｜dy/dx｜x=0 =0 ,选B