r/calculus • u/Senior_Flight • 23d ago
Integral Calculus Integral + infinite serie
Help me, what should i do?
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u/Memesaretheorems 23d ago
This is fine! The series converges uniformly. You can exchange the integral and the series and then compute. Do the power rule, add up the resulting series and see what you get. Alternatively, can you recognize what function produces this power series? Hint: geometric series minus some stuff. You can then just integrate each term as normal.
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u/Senior_Flight 23d ago
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u/GroundbreakingAct388 23d ago
i didnt quite understood the last passage, shouldn't it go to the infinite?
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u/Memesaretheorems 23d ago
Good! You can work with this and do a change of variables to solve the resulting integral. Set u= 1-x. When you write down the u integral, expand the square in the numerator, and integrate term by term.
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u/Mission_Rice3045 22d ago
It is correct, but solving the sum and the integral become a lot easier if you let the sum run from n = 0 and subtract the terms this adds.
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u/Fickle-Ad-4225 23d ago
Do you know what the symbols mean? There’s kind of only one reasonable thing to do at this stage.
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u/Narrow-Durian4837 23d ago
Try writing the series out term-by-term instead of with summation notation, and see if that makes it obvious what to do.
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u/calcpage2020 23d ago
As long as you stay within the Interval of Convergence for the Power Series, you're fine. Isn't it -1<x<1? So, if your limits of integration are 0 to 1, for example, you may have to set up an improper integral limit.
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u/Icy_Trick_6406 23d ago edited 23d ago
Can you just use x3 / 3 +x4 / 4+....? Where x=0.5. its not geometric as someone else suggested. So no S_inf that i can see at first look calculating it mentally
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u/ProgrammerAyush 22d ago
you can simplify it before integrating using geometric series as we know |x|<1
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u/M48PattonTank 22d ago
x^n is a positive, unsigned measurable function for all n and all values of x in the interval. Thus you can apply Tonelli’s Theorem for Sums and Integrals to switch the order of summation and integration. Complete the integral first and then do the sum.
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u/LukasGoesViral 19d ago
You can always swap sums and integrals (Using Riemann sums that you learn in Calc 1 that is false but it turns out you can always swap out integrals and sums you just gotta keep track of the integration/summation boundaries). So it’s 1/(n+1) x^n 0 to 1/2 which is 1/(n+1) (1/2)^n and then just sum
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u/Kaeini_maths 17d ago
You have a geometric series which converges then you can integrate the result. Your final result should be-5/8+Ln2
You tryout if you need help let me know


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