We are given that $(2 + 4x^2) \le f(x)$. We need to find a correct estimate for the integral $\int_0^2 f(x) \, dx$.

AnalysisIntegrationInequalitiesDefinite IntegralsCalculus
2025/5/10

1. Problem Description

We are given that (2+4x2)f(x)(2 + 4x^2) \le f(x). We need to find a correct estimate for the integral 02f(x)dx\int_0^2 f(x) \, dx.

2. Solution Steps

Since (2+4x2)f(x)(2 + 4x^2) \le f(x), we can integrate both sides of the inequality from 0 to 2:
02(2+4x2)dx02f(x)dx\int_0^2 (2 + 4x^2) \, dx \le \int_0^2 f(x) \, dx
Now, let's evaluate the integral on the left side:
02(2+4x2)dx=022dx+024x2dx\int_0^2 (2 + 4x^2) \, dx = \int_0^2 2 \, dx + \int_0^2 4x^2 \, dx
=202dx+402x2dx= 2 \int_0^2 dx + 4 \int_0^2 x^2 \, dx
=2[x]02+4[x33]02= 2[x]_0^2 + 4 \left[ \frac{x^3}{3} \right]_0^2
=2(20)+4(233033)= 2(2 - 0) + 4 \left( \frac{2^3}{3} - \frac{0^3}{3} \right)
=2(2)+4(83)= 2(2) + 4 \left( \frac{8}{3} \right)
=4+323= 4 + \frac{32}{3}
=123+323= \frac{12}{3} + \frac{32}{3}
=443= \frac{44}{3}
Since 44314.6667\frac{44}{3} \approx 14.6667, we have
44302f(x)dx\frac{44}{3} \le \int_0^2 f(x) \, dx
Therefore, 02f(x)dx44314.6667\int_0^2 f(x) \, dx \ge \frac{44}{3} \approx 14.6667.
Comparing this with the given options:
A. 02f(x)dx14.67\int_0^2 f(x) \, dx \ge 14.67
B. 02f(x)dx14.67\int_0^2 f(x) \, dx \le 14.67
C. 02f(x)dx2\int_0^2 f(x) \, dx \le 2
D. 02f(x)dx0\int_0^2 f(x) \, dx \ge 0
Since 44314.6667\frac{44}{3} \approx 14.6667, and 14.6667<14.6714.6667 < 14.67, it follows that 02f(x)dx14.6667\int_0^2 f(x) dx \ge 14.6667, which is very close to 14.
6

7. Also, $f(x) \ge 2 + 4x^2 \ge 2$, therefore $\int_0^2 f(x) dx \ge \int_0^2 2 dx = 4$. Therefore, option C can be ruled out. Option D is too weak of a bound, as $f(x)$ must be positive on the interval $[0, 2]$. Option B is not true, as $\int_0^2 f(x) dx \ge \frac{44}{3}$. Since $\frac{44}{3} \approx 14.6667 < 14.67$, it follows that $\int_0^2 f(x) dx \ge 14.6667$. We also know that $\int_0^2 f(x) dx \ge 14.67$ can be false, but the problem asks which of the following is a correct estimate. The integral is greater than or equal to $\frac{44}{3}$, which is about 14.67, so option A is correct.

3. Final Answer

A. 02f(x)dx14.67\int_0^2 f(x) \, dx \ge 14.67

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