Double Integral Help: Solving with u-sub

In summary, a double integral is a mathematical operation used to find the volume under a surface in three-dimensional space. U-substitution is a technique that can be used to simplify the integration process of functions. To solve a double integral using u-substitution, one must identify the inner and outer functions, substitute a new variable, and evaluate the integral using integration rules. Some benefits of u-substitution include simplifying complex integrals and utilizing the fundamental theorem of calculus. However, it may not work for all integrals and may not be possible to find a suitable substitution in some cases.
  • #1
UrbanXrisis
1,196
1
[tex]\int _0 ^{\pi/3} \int _0 ^{\pi/4} x cos(x+y) dy dx[/tex]
[tex]\int _0 ^{\pi/3} xsin(x+\frac{\pi}{4}) dx [/tex]
using u-sub, u=x, dv=sin(x+pi/4)

[tex]-xcos\left(x+\frac{\pi}{4}\right)+ sin\left(x+\frac{\pi}{4}\right)-sin\left(\frac{\pi}{4}\left) |_0^{\pi/3} [/tex]

[tex]-\frac{\pi}{3}cos\left(\frac{\pi}{3}+\frac{\pi}{4}\right)+ sin\left(\frac{\pi}{3}+\frac{\pi}{4}\right)-sin\left(\frac{\pi}{4}\right) [/tex]

i don't know where I made the mistake
 
Last edited:
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  • #2
Don't forgot the lower limit in the first integration: [itex]\sin(x+\pi/4)-\sin(x)[/itex]
 

Related to Double Integral Help: Solving with u-sub

1. What is a double integral?

A double integral is a type of mathematical operation used in calculus to find the volume under a surface in three-dimensional space. It involves integrating a function of two variables over a specified region in the x-y plane.

2. What is a u-substitution?

U-substitution, also known as the substitution method, is a technique used to simplify the integration of functions. It involves substituting a new variable, u, for an expression within the original function in order to make the integration process more manageable.

3. How do I solve a double integral using u-substitution?

To solve a double integral using u-substitution, follow these steps:1. Identify the inner and outer functions in the integrand.2. Let u be the inner function and set it equal to a new variable.3. Calculate the differential of u, du.4. Substitute the new variable and its differential in the integrand.5. Rewrite the limits of integration in terms of u.6. Evaluate the integral using the rules of integration.7. Substitute the original variable back into the final answer.

4. What are the benefits of using u-substitution to solve double integrals?

U-substitution can simplify the integration process and make it easier to solve complex integrals. It also allows for the use of the fundamental theorem of calculus, which states that the derivative of an integral is the original function. This can be useful when evaluating indefinite integrals.

5. Are there any limitations to using u-substitution for double integrals?

Yes, u-substitution may not work for all integrals. It is most effective when the inner function is a simple expression and the outer function is more complex. Additionally, it may not be possible to find a suitable substitution for some integrals, in which case other integration techniques may need to be used.

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