Question

设y=e^(-x)cos(3-x),求dy 详细些

Answer 1

∵dy/dx=y'=[e^(-x)]'cos(3-x)+e^(-x)[cos(3-x)]'

=-e^(-x)cos(3-x)+e^(-x)(sin(3-x)

=[-cos(3-x)+(sin(3-x)]e^(-x)

∴dy=e^(-x)[-cos(3-x)+(sin(3-x)]dx