1. y=sinx-cosx,求y'|x=π/6
y'=cosx + sinx
y'|x=π/6 = (√3)/2 + 1/2
2. y^2 -2xy+9=0,求dy/dx
直接对方程求关于x的导数：2y* y' - 2（y + xy')=0
整理,得 y' = y/(y-x)
3. y=xsin2x,求dy
y' = sin2x + 2xcos2x
dy = y' dx = (sin2x + 2xcos2x)dx