y=tan(x+y)求dy
问题描述：
y=tan(x+y)求dy
答
dy＝d[tan(x+y)]＝(dx+dy)/[cos(x+y)]^2
即
dy*[cos(x+y)]^2＝dx+dy,再移项,得
-dy*[sin(x+y)]^2＝dx
所以
dy＝-dx/[sin(x+y)]^2