Limit Function Derivative F5287F
1. **State the problem:** We are given the limit $$\lim_{x \to 1} \frac{f^3(x) - 8}{2x^3 - x - 1} = 16$$ and the function $$y = f(x)$$. We want to find information about the function at $$x=1$$, such as the value of $$f(1)$$ and possibly the derivative $$f'(1)$$.
2. **Recall the limit definition and factorization:** The numerator is $$f^3(x) - 8$$ which is a difference of cubes. Recall the formula:
$$a^3 - b^3 = (a - b)(a^2 + ab + b^2)$$
Here, $$a = f(x)$$ and $$b = 2$$ because $$8 = 2^3$$.
3. **Apply the difference of cubes factorization:**
$$f^3(x) - 8 = (f(x) - 2)(f^2(x) + 2f(x) + 4)$$
4. **Analyze the denominator at $$x=1$$:**
Calculate $$2(1)^3 - 1 - 1 = 2 - 1 - 1 = 0$$.
5. **Since both numerator and denominator approach 0 at $$x=1$$, this is an indeterminate form $$\frac{0}{0}$$, so the limit can be evaluated by factoring or using derivatives (L'Hôpital's Rule). But first, note that for the numerator to be zero at $$x=1$$, we must have:
$$f^3(1) - 8 = 0 \implies f^3(1) = 8 \implies f(1) = 2$$.
6. **Rewrite the limit using the factorization:**
$$\lim_{x \to 1} \frac{(f(x) - 2)(f^2(x) + 2f(x) + 4)}{2x^3 - x - 1} = 16$$
7. **Since $$f^2(x) + 2f(x) + 4$$ is continuous at $$x=1$$, substitute $$f(1) = 2$$:
$$f^2(1) + 2f(1) + 4 = 2^2 + 2 \times 2 + 4 = 4 + 4 + 4 = 12$$
8. **Define $$g(x) = 2x^3 - x - 1$$ and note $$g(1) = 0$$. The limit becomes:
$$\lim_{x \to 1} \frac{(f(x) - 2) \times 12}{g(x)} = 16$$
9. **Rearranged:**
$$\lim_{x \to 1} \frac{f(x) - 2}{g(x)} = \frac{16}{12} = \frac{4}{3}$$
10. **Since $$f(1) = 2$$ and $$g(1) = 0$$, the limit of $$\frac{f(x) - f(1)}{g(x) - g(1)}$$ as $$x \to 1$$ is $$\frac{4}{3}$$. This is the definition of the derivative of $$f$$ with respect to $$g$$ at $$x=1$$:
$$\frac{df}{dg}\bigg|_{x=1} = \frac{4}{3}$$
11. **Find $$g'(x)$$:**
$$g'(x) = 6x^2 - 1$$
At $$x=1$$:
$$g'(1) = 6(1)^2 - 1 = 6 - 1 = 5$$
12. **Use the chain rule:**
$$\frac{df}{dx} = \frac{df}{dg} \times \frac{dg}{dx}$$
At $$x=1$$:
$$f'(1) = \frac{4}{3} \times 5 = \frac{20}{3}$$
**Final answers:**
- $$f(1) = 2$$
- $$f'(1) = \frac{20}{3}$$