Taylor Local Minimum
1. **State the problem:** We want to find the Taylor series expansion of the function $$f(x) = e^{\sin(x^4)} \cos(x^2)$$ about $$x=0$$ and determine the nature of the critical point at $$x=0$$.
2. **Recall Taylor series expansions:**
- $$\sin(x) = x - \frac{x^3}{6} + O(x^5)$$
- $$e^x = 1 + x + \frac{x^2}{2} + O(x^3)$$
- $$\cos(x) = 1 - \frac{x^2}{2} + \frac{x^4}{24} + O(x^6)$$
3. **Expand inner functions:**
- $$\sin(x^4) = x^4 - \frac{x^{12}}{6} + O(x^{20})$$
4. **Expand $$e^{\sin(x^4)}$$:**
Using $$e^y = 1 + y + \frac{y^2}{2} + O(y^3)$$ with $$y = \sin(x^4)$$,
$$e^{\sin(x^4)} = 1 + x^4 + \frac{x^8}{2} + O(x^{12})$$
5. **Expand $$\cos(x^2)$$:**
$$\cos(x^2) = 1 - \frac{x^4}{2} + \frac{x^8}{24} + O(x^{12})$$
6. **Multiply expansions:**
$$f(x) = e^{\sin(x^4)} \cos(x^2) = \left(1 + x^4 + \frac{x^8}{2} + O(x^{12})\right) \left(1 - \frac{x^4}{2} + \frac{x^8}{24} + O(x^{12})\right)$$
Multiply terms up to order $$x^8$$:
$$= 1 + x^4 - \frac{x^4}{2} + \frac{x^8}{2} + \frac{x^8}{24} - \frac{x^8}{2} + O(x^{12})$$
Simplify:
$$= 1 + \frac{x^4}{2} + \left(\frac{1}{2} + \frac{1}{24} - \frac{1}{2}\right) x^8 + O(x^{12}) = 1 + \frac{x^4}{2} + \frac{x^8}{24} + O(x^{12})$$
7. **Interpretation:**
- The Taylor series about $$x=0$$ is $$1 + \frac{x^4}{2} + O(x^5)$$.
- The first derivative at $$x=0$$ is zero (no $$x$$ or $$x^3$$ terms).
- The second derivative at $$x=0$$ is zero (no $$x^2$$ term).
- The first nonzero term after the constant is $$\frac{x^4}{2}$$, which is positive.
8. **Conclusion:**
Since the first nonzero derivative after the constant term is the fourth derivative and it is positive, $$x=0$$ is a local minimum.
**Final answer:** The correct statement is:
"Its Taylor series expansion about $$x=0$$ is $$1 + \frac{x^4}{2} + O(x^5)$$. Hence it has a local minimum at $$x=0$$."