Substitution of a definite integral

In summary, the speaker is questioning why the √v disappears in a particular step, and also expressing confusion about the 20/3 value that appears. The expert explains that the √v disappears because of a substitution made for dv, and clarifies that the 20/3 value comes from a previous step and not from integration.
  • #1
bobsmith76
336
0

Homework Statement



Screenshot2012-02-04at91000PM.png


Screenshot2012-02-04at90922PM.png


The Attempt at a Solution



I don't see why the √v disappears in this step

Screenshot2012-02-04at91249PM.png


I understand how they got the 20/3, because they integrated, but if they're integrating then I would think v^-1/2 would become (v^1/2)/(1/2)
 
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  • #2
bobsmith76 said:

Homework Statement



Screenshot2012-02-04at91000PM.png


Screenshot2012-02-04at90922PM.png


The Attempt at a Solution



I don't see why the √v disappears in this step

Screenshot2012-02-04at91249PM.png


I understand how they got the 20/3, because they integrated, but if they're integrating then I would think v^-1/2 would become (v^-1/2)/(1/2)

The √v disappears because [itex]dv = \frac{2}{3}v^{-\frac{1}{2}}du[/itex] (rearranging the equation they gave for du). Substitute that for dv into the integrand and you'll see why. The 20/3 comes from 10*(2/3) during this substitution, NOT from integration (which hasn't occurred yet).
 

Related to Substitution of a definite integral

1. What is the purpose of substitution in a definite integral?

The purpose of substitution in a definite integral is to simplify the integrand and make it easier to evaluate. This is done by replacing the variable in the integrand with a new variable, and then using the appropriate substitution rule to rewrite the integral in terms of the new variable.

2. How do you choose the substitution variable?

The substitution variable is typically chosen to be a function of the variable in the original integrand. It should also be chosen in a way that will simplify the integrand and make it easier to evaluate. Common choices for substitution variables include u, x^2, sin(x), or e^x.

3. What is the general process for substitution in a definite integral?

The general process for substitution in a definite integral is as follows:

  • Identify the substitution variable and make the appropriate substitution in the integrand.
  • Use the appropriate substitution rule to rewrite the integral in terms of the substitution variable.
  • Integrate the new integral using the standard integration techniques.
  • Replace the substitution variable with the original variable to obtain the final answer.

4. Are there any restrictions on when substitution can be used in a definite integral?

Yes, there are certain restrictions on when substitution can be used in a definite integral. The most important restriction is that the substitution must be a one-to-one function. This means that each value of the substitution variable must correspond to exactly one value of the original variable. Additionally, the limits of integration must also be adjusted to reflect the change in variables.

5. Can substitution be used to solve all definite integrals?

No, substitution cannot be used to solve all definite integrals. There are certain integrals that cannot be simplified using substitution, and may require other integration techniques such as integration by parts or trigonometric identities. It is important to carefully consider the integrand and choose the appropriate method for evaluation.

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