There are a lot of pies here (My tenth integral problem)

Calculus Level 4

If you evaluate 0 π / 2 x 2016 sin x d x \displaystyle \int_{0}^{\pi/2} x^{2016} \sin x \, dx , you'll get a sum of different powers of π \pi 's multiplied by different coefficients.

What is the highest exponent of π \pi you will encounter in the evaluation of the integral, assuming the evaluation you get is in exact form, and that you leave everything expanded with no factoring of any kind done? Enter this exponent as your answer.

Bonus question : How could this be generalized? What happens if you do this with cosine? Why does this work?


The answer is 2015.

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1 solution

Hamza A
Feb 9, 2016

by tabular integration,we can notice a pattern

x n sin ( x ) = c o s ( x ) k = 0 n 2 ( 1 ) k + 1 n ! x n 2 k ( n 2 r ) ! + sin ( x ) k = 0 n 1 2 ( 1 ) k n ! x n 2 k 1 ( n 2 k 1 ) ! \displaystyle\int { x^{ n }\sin { (x) } } =cos(x)\displaystyle\sum _{ k=0 }^{ \left\lfloor \frac { n }{ 2 } \right\rfloor }{ (-1)^{ k+1 }\frac { n!x^{ n-2k } }{ (n-2r)! } } +\sin (x)\displaystyle\sum _{ k=0 }^{ \left\lfloor \frac { n-1 }{ 2 } \right\rfloor }{ (-1)^{ k } } \frac { n!{ x }^{ n-2k-1 } }{ (n-2k-1)! } \\ \\

evaluating the integral at x= π 2 \frac{\pi}{2} we get the highest power is from the second sum which is 2016-1= 2015 \boxed{2015} then evaluating it at x=0 we get 0 since x is in both sums it zeros every term in the sum so the answer is

2015 \boxed{2015}

nice problem!

Bonus :

we can derive the general formula for cosine using the same method

it is

x n cos ( x ) d x = sin ( x ) i = 0 n 2 ( 1 ) i n ! x m 2 i ( n 2 i ) ! + cos ( x ) i = 0 n 1 2 ( 1 ) i n ! x n 1 2 i ( n 2 i 1 ) ! \displaystyle\int { x^{ n }\cos { (x) } dx } =\sin { (x) } \displaystyle\sum _{ i=0 }^{ \left\lfloor \frac { n }{ 2 } \right\rfloor }{ (-1)^{ i } } \frac { n!x^{ m-2i } }{ (n-2i)! } +\cos (x)\displaystyle\sum _{ i=0 }^{ \left\lfloor \frac { n-1 }{ 2 } \right\rfloor }{ (-1)^{ i } } \frac { n!{ x }^{ n-1-2i } }{ (n-2i-1)! }

so in this case the highest power is 2016,or generally, n n

this works because of integration by parts.I think that there may be a combinatoric explanation since i see factorials and even more so ,i think that there may be a geometrical interpretation.If anyone wants to do another approach,that would be awesome because i am eager to see it :)

Great solution! Thanks :)

Hobart Pao - 5 years, 4 months ago

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thanks for making awesome problems! :)

Hamza A - 5 years, 4 months ago

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