If f (x) is a differentiable function, then dy is a linear function of △ X Why? Dy = f '(x) △ x, f (x) is a function of X, also called the linear function of △ x?

If f (x) is a differentiable function, then dy is a linear function of △ X Why? Dy = f '(x) △ x, f (x) is a function of X, also called the linear function of △ x?

You should note that now x is fixed and the variable is △ x, so from dy = f '(x) △ x, you can know that dy is a linear function of △ X