Dy refers to the change of tangent at this point when the change of X is extremely small, that is, the corresponding value of X, while Y of triangle refers to the change of function value. The difference between the two is that the variation of x is infinitely small, and when y=x, it is equal everywhere. When y = x 2, there are still some differences in graphics. Because the change of x is small, it is equal by default in most cases.