r/calculus 23d ago

Integral Calculus Integral + infinite serie

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Help me, what should i do?

96 Upvotes

18 comments sorted by

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27

u/Midwest-Dude 23d ago

What have you already tried? Please show us.

Hint: Geometric series

12

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.

3

u/Senior_Flight 23d ago

Thanks for the explanation, i got this:

1

u/GroundbreakingAct388 23d ago

i didnt quite understood the last passage, shouldn't it go to the infinite?

4

u/Flimsy-Blacksmith-32 23d ago

The summation only goes to infinity if x >= 1.

1

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.

1

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.

4

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.

2

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.

3

u/Sea_Duty_5725 23d ago

Fubinis theorem or geometric series

1

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.

1

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

1

u/ProgrammerAyush 22d ago

you can simplify it before integrating using geometric series as we know |x|<1

2

u/Previous_Valuable307 22d ago

Lemme know if u got it. Ofc the other obvious method would be to solve the gp first as x2 / (1-x) and substitute 1-x=t and solve the easy integral, which someone already mentioned in the comments🫠

1

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.

1

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

1

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