Question: Question 8
Use the alternative form of the derivative to find the derivative of the function $$f(x) = x^3 - 2$$ at $$x = 3$$.
Option A
$$f'(3) = 81$$
Option B
$$f'(3) = 9$$
Option C
$$f'(3) = 27$$
Option D
$$f'(3) = 25$$
Option E
$$f'(3) = 243$$
1. **State the problem:** We need to find the derivative of the function $$f(x) = x^3 - 2$$ at $$x = 3$$ using the alternative form of the derivative.
2. **Recall the alternative form of the derivative:**
$$
f'(a) = \lim_{h \to 0} \frac{f(a+h) - f(a)}{h}
$$
where $$a = 3$$ in this problem.
3. **Calculate $$f(3)$$:**
$$
f(3) = 3^3 - 2 = 27 - 2 = 25
$$
4. **Calculate $$f(3+h)$$:**
$$
f(3+h) = (3+h)^3 - 2
$$
Expand using binomial theorem:
$$
(3+h)^3 = 27 + 27h + 9h^2 + h^3
$$
So,
$$
f(3+h) = 27 + 27h + 9h^2 + h^3 - 2 = 25 + 27h + 9h^2 + h^3
$$
5. **Form the difference quotient:**
$$
\frac{f(3+h) - f(3)}{h} = \frac{(25 + 27h + 9h^2 + h^3) - 25}{h} = \frac{27h + 9h^2 + h^3}{h}
$$
6. **Simplify the fraction by canceling $$h$$:**
$$
\frac{\cancel{h}(27 + 9h + h^2)}{\cancel{h}} = 27 + 9h + h^2
$$
7. **Take the limit as $$h \to 0$$:**
$$
f'(3) = \lim_{h \to 0} (27 + 9h + h^2) = 27
$$
8. **Conclusion:** The derivative of $$f(x)$$ at $$x=3$$ is $$27$$.
**Final answer:** Option C: $$f'(3) = 27$$