I've got the following review problem in my micro-econ course:
y = f(x) = 5x^2 - 4x
(a) Find delta-y/delta-x as a of x and delta-x.
(b) Find dy/dx.
(b)'s just the derivative and I've confirmed the answer in the book. However, I cannot work out how one arrives at this solution for (a): 10x + (5delta-x) - 4
Where does the 5delta-x come from?
The text gives this form:
delta-y/delta-x = [f(x - delta-x) - f(x)] / delta-x
Using that, I got 5delta-x - 6
Any help would be greatly appreciated. Thanks in advance.