Chapter 4: Problem 40
Evaluate the integral. $$ \int_{1}^{5} \sqrt{2 x-1} d x $$
Short Answer
Expert verified
The value of the definite integral \(\int_{1}^{5} \sqrt{2 x-1} dx\) is \(\frac{26}{3}\).
Step by step solution
01
Apply substitution
Let's set:
$$
u = 2x - 1
$$
Now differentiate u with respect to x:
$$
\frac{du}{dx} = 2
$$
We can rewrite the differential dx as follows:
$$
dx = \frac{du}{2}
$$
Now, we also need to adjust our integral limits. When \(x = 1\), we have:
$$
u = 2(1)-1 = 1
$$
And when \(x = 5\), we have:
$$
u = 2(5)-1 = 9
$$
So the integral becomes:
$$
\int_{1}^{5} \sqrt{2 x-1} dx = \frac{1}{2} \int_{1}^{9} \sqrt{u} du
$$
02
Apply the power rule for integration
We can rewrite the integral as a power function:
$$
\frac{1}{2} \int_{1}^{9} u^{1/2} du
$$
Now we can apply the power rule for integration, which states that:
$$
\int x^n dx = \frac{x^{n+1}}{n+1} + C
$$
In our case, n is 1/2, therefore:
$$
\frac{1}{2} \left[ \frac{u^{3/2}}{\frac{3}{2}} \right]_{1}^{9}
$$
Simplify the expression and evaluate the integral at the limits:
$$
\frac{1}{3} \left[ u^{3/2} \right]_{1}^{9} = \frac{1}{3} (9^{3/2} - 1^{3/2})
$$
03
Simplify and compute the final result
Now we can compute the result:
$$
\frac{1}{3} (27 - 1) = \frac{1}{3} (26) = \frac{26}{3}
$$
So the value of the definite integral is:
$$
\int_{1}^{5} \sqrt{2 x-1} dx = \frac{26}{3}
$$
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Substitution Method
The substitution method is a powerful technique used in calculus to simplify the process of integration, especially when dealing with complex functions. It is also often referred to as 'u-substitution'. In the original problem, substitution was used to transform the integral of \( \sqrt{2x-1} \) into a simpler form. Let's break it down:
- First, identify a part of the integral to replace with a new variable. In this case, the expression \( 2x - 1 \) was substituted with \( u \).
- Differentiate \( u \) to find \( \frac{du}{dx} = 2 \), which helps you solve for \( dx \), getting \( dx = \frac{du}{2} \).
- The limits of the integral must also change based on the substitution. When \( x = 1 \), \( u = 1 \); and when \( x = 5 \), \( u = 9 \).
Power Rule for Integration
The power rule for integration is an essential tool used when integrating expressions of the form \( x^n \). Often, after simplifying an integral using substitution, the expression is rewritten in a way that allows the power rule to be applied directly.
- The general form of the power rule is \( \int x^n \, dx = \frac{x^{n+1}}{n+1} + C \), where \( n eq -1 \).
- In the given solution, the original integral \( \sqrt{u} \) was rewritten as \( u^{1/2} \) to apply the power rule.
- Here, \( n = 1/2 \). Therefore, applying the rule gives \( \int u^{1/2} \, du = \frac{u^{3/2}}{3/2} + C \).
Integral Limits
Integral limits refer to the boundaries at which the definite integral is evaluated. For definite integrals, these limits transform the process from finding indefinite integrals (which include a constant \( C \)) to computing an exact number.
- Once substitution is made, the original limits need updating to match the new variable. This is crucial since keeping old limits would mean integrating along the wrong path.
- Initially, the integral had limits \( 1 \) to \( 5 \), based on \( x \). When changing to \( u \), using \( u = 2x - 1 \), these transformed to \( u = 1 \) and \( u = 9 \).
- Finally, to evaluate the definite integral \( \int_{1}^{9} \frac{u^{3/2}}{3/2} \, du \), compute \( F(9) - F(1) \) where \( F(u) \) is the antiderivative found from integration.