When should we use partial derivatives and when should we use ordinary derivatives? Please explain in 1 or 2 paragraph Thank you. Solution From a particular point of view total derivative and partial derivatives are the same. The dependencies of the variables and the nature of derivatives should be clearly stated, but it is often they are not. One may make the convention that when the total notation is used, variables are dependent unless otherwise stated; and when partial notation is used they are independent unless otherwise stated. The dy/dx = 0 assumption is not incorrect. The user that does not adust for it is incorrect. If a calculator given 3*4+3 returns 15 but the user intended 3*(4+3) which returns 21 the user was in error..