Question 183 of 480
Evaluate \(\int^{2} _{3}(x^2 - 2x)dx\)
Correct Answer:
C
Explanation
To evaluate the integral \(\int^{2} _{3}(x^2 - 2x)dx\), we will follow a systematic approach. Let's break it down step-by-step.
### Step 1: Understand the Integral
The integral \(\int^{2} _{3}(x^2 - 2x)dx\) represents the area under the curve of the function \(f(x) = x^2 - 2x\) from \(x = 3\) to \(x = 2\). However, notice that the limits of integration are from 3 to 2, which means we are integrating in the reverse direction. This will affect the sign of our result.
### Step 2: Find the Antiderivative
To evaluate the integral, we first need to find the antiderivative of the function \(f(x) = x^2 - 2x\).
1. **Antiderivative Calculation**:
- The antiderivative of \(x^2\) is \(\frac{x^3}{3}\).
- The antiderivative of \(-2x\) is \(-x^2\).
- Therefore, the antiderivative \(F(x)\) of \(f(x)\) is:
\[
F(x) = \frac{x^3}{3} - x^2 + C
\]
where \(C\) is the constant of integration (not needed for definite integrals).
### Step 3: Evaluate the Definite Integral
Now we will evaluate the definite integral using the Fundamental Theorem of Calculus, which states that:
\[
\int_a^b f(x) \, dx = F(b) - F(a)
\]
In our case, we need to evaluate \(F(2)\) and \(F(3)\):
1. **Calculate \(F(2)\)**:
\[
F(2) = \frac{2^3}{3} - 2^2 = \frac{8}{3} - 4 = \frac{8}{3} - \frac{12}{3} = -\frac{4}{3}
\]
2. **Calculate \(F(3)\)**:
\[
F(3) = \frac{3^3}{3} - 3^2 = \frac{27}{3} - 9 = 9 - 9 = 0
\]
3. **Apply the Fundamental Theorem**:
Now we substitute these values into the formula:
\[
\int^{2}_{3}(x^2 - 2x)dx = F(2) - F(3) = -\frac{4}{3} - 0 = -\frac{4}{3}
\]
### Step 4: Adjust for the Limits of Integration
Since we integrated from 3 to 2, we need to reverse the limits, which changes the sign of the result:
\[
\int^{2}_{3}(x^2 - 2x)dx = -\left(-\frac{4}{3}\right) = \frac{4}{3}
\]
### Conclusion: Final Answer
Thus, the value of the integral \(\int^{2}_{3}(x^2 - 2x)dx\) is \(\frac{4}{3}\).
### Explanation of Options
- **Option A (4)**: This is incorrect because the area calculated does not equal 4.
- **Option B (2)**: This is incorrect as the area calculated does not equal 2.
- **Option C (4/3)**: This is the correct answer as derived from our calculations.
- **Option D (1/3)**: This is incorrect because the area calculated does not equal 1/3.
### Revision Summary
- The integral \(\int^{2}_{3}(x^2 - 2x)dx\) was evaluated using the antiderivative.
- The antiderivative of \(x^2 - 2x\) is \(\frac{x^3}{3} - x^2\).
- The definite integral was calculated as \(-\frac{4}{3}\) and adjusted for the limits of integration.
- The final answer is \(\frac{4}{3}\), corresponding to option C.