已知y=cos(x+y),求dy
问题描述：
已知y=cos(x+y),求dy
答
dy=d(cos(x+y))
dy=-sin(x+y)d(x+y)
dy+sin(x+y)(dx+dy)=0
(1+sin(x+y))dy=-sin(x+y)dx
dy=-sin(x+y)dx/(1+sin(x+y))