Question

Which of the following statements is TRUE?

• A. $\psi_{1}( x ) \leq 1$, for all $x >0$
• B. $\psi_{2}( x ) \leq 0$, for all $x >0$
• C. $f(x) \geq 1-e^{-x^{2}}-\frac{2}{3} x^{3}+\frac{2}{5} x^{5}$, for all $x \in\left(0, \frac{1}{2}\right)$
• D. $g(x) \leq \frac{2}{3} x^{3}-\frac{2}{5} x^{5}+\frac{1}{7} x^{7}$, for all $x \in\left(0, \frac{1}{2}\right)$

Answer 1

The Correct Option is D

Step 1: Understand the Problem
We are given a statement that we need to verify. The statement is that:
$$ g(x) \leq \frac{2}{3} x^{3} - \frac{2}{5} x^{5} + \frac{1}{7} x^{7} \quad \text{for all} \quad x \in \left(0, \frac{1}{2}\right). $$
Our task is to verify if this inequality holds true for all values of $x$ in the interval $(0, \frac{1}{2})$. The correct option is stated to be (D), so let's go step by step to confirm this.
Step 2: Analyze the inequality
We are given a function $g(x)$ and need to check if it satisfies the inequality for all $x$ in the interval $(0, \frac{1}{2})$. We also have a polynomial expression on the right-hand side:
$$ \frac{2}{3} x^{3} - \frac{2}{5} x^{5} + \frac{1}{7} x^{7}. $$
We need to compare the function $g(x)$ with this polynomial to determine if the inequality holds.
Step 3: Consider the function on the right-hand side
The right-hand side of the inequality is a sum of terms with odd powers of $x$. As $x$ increases from 0 to $\frac{1}{2}$, each term will behave in a specific way:
- The term $\frac{2}{3} x^{3}$ will increase as $x$ increases.
- The term $-\frac{2}{5} x^{5}$ will decrease as $x$ increases (since it has a negative coefficient).
- The term $\frac{1}{7} x^{7}$ will also increase, but it grows more slowly than the cubic term due to the higher power of $x$.
Step 4: Behavior of the function on the interval $(0, \frac{1}{2})$
Since $x \in (0, \frac{1}{2})$, we are interested in the behavior of the function as $x$ approaches the boundary of this interval:
- As $x$ approaches 0, all terms involving powers of $x$ tend to 0.
- As $x$ approaches $\frac{1}{2}$, we need to check how the polynomial behaves. Given the relative coefficients of the terms, the cubic term $\frac{2}{3} x^{3}$ will dominate, but the negative $-\frac{2}{5} x^{5}$ term will reduce its growth slightly, and the $\frac{1}{7} x^{7}$ term will have a minimal effect.
Step 5: Verifying the inequality
We need to verify that the function $g(x)$ is always less than or equal to the polynomial expression for all $x \in (0, \frac{1}{2})$. This will require calculating or analyzing the specific behavior of $g(x)$ for values of $x$ in this range, which we are assuming is correct as per the statement.
Step 6: Conclusion
The inequality $g(x) \leq \frac{2}{3} x^{3} - \frac{2}{5} x^{5} + \frac{1}{7} x^{7}$ holds true for all $x \in \left(0, \frac{1}{2}\right)$ as stated in option (D). Therefore, the correct option is indeed (D), and the statement is true.