Double Integral Help: Solving with u-sub

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SUMMARY

The discussion focuses on solving the double integral \(\int _0 ^{\pi/3} \int _0 ^{\pi/4} x \cos(x+y) \, dy \, dx\) using u-substitution. The user attempted to apply u-substitution with \(u=x\) and \(dv=\sin(x+\frac{\pi}{4})\), but encountered an error in the integration limits. The correct evaluation involves ensuring the lower limit is included in the first integration, leading to the expression \(\sin(x+\pi/4)-\sin(x)\). The final evaluation of the integral requires careful attention to these limits to avoid mistakes.

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Students and educators in calculus, mathematicians focusing on integral calculus, and anyone looking to improve their skills in evaluating double integrals and applying u-substitution techniques.

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[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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Don't forgot the lower limit in the first integration: [itex]\sin(x+\pi/4)-\sin(x)[/itex]
 

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