满足线性 [u(x)±v(x)]′=u′(x)±v′(x) 2. [u(x)⋅v(x)]′=u′(x)v(x)+u(x)v′(x) 3. [v(x)u(x)]′=v2(x)u′(x)⋅v(x)−u(x)v′(x)