Isn't dy equal to △y

Dy represents the slope of x0, and △y is the difference of y in an interval. It's different.

In case of such a problem, just use a special function to verify it. It's a multiple-choice question anyway.

For example, f(x)=x? This function, f'' (x) = 2 >; 0, f'(x)=2x, when x

dy = f '(- 1)= 2×(- 1)=-2

Choose B.