Don't Take My Seat at the PolyExpo

Calculus Level 4

1 1 ( 18 x 8 + 48 x 7 + 12 x 6 + 36 x 5 + 9 x 2 + 16 x + 2 ) e x 6 1 d x = ? \int_{-1}^1 (18x^8+48x^7+12x^6+36x^5+9x^2+16x+2)e^{x^6-1}dx = \ ?


The answer is 10.

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1 solution

James Wilson
Jan 10, 2021

Set

( 18 x 8 + 48 x 7 + 12 x 6 + 36 x 5 + 9 x 2 + 16 x + 2 ) e x 6 1 = d d x [ ( a x 3 + b x 2 + c x + d ) e x 6 1 ] (18x^8+48x^7+12x^6+36x^5+9x^2+16x+2)e^{x^6-1}=\frac{d}{dx}\Big[ (ax^3+bx^2+cx+d)e^{x^6-1}\Big] .

= ( 3 a x 2 + 2 b x + c ) e x 6 1 + ( 6 a x 8 + 6 b x 7 + 6 c x 6 + 6 d x 5 ) e x 6 1 =(3ax^2+2bx+c)e^{x^6-1}+(6ax^8+6bx^7+6cx^6+6dx^5)e^{x^6-1}

= ( 6 a x 8 + 6 b x 7 + 6 c x 6 + 6 d x 5 + 3 a x 2 + 2 b x + c ) e x 6 1 . =(6ax^8+6bx^7+6cx^6+6dx^5+3ax^2+2bx+c)e^{x^6-1}.

Equating coefficients yields

a = 3 , b = 8 , c = 2 , d = 6. a=3, b=8, c=2, d=6.

So, the antiderivative is

( 3 x 3 + 8 x 2 + 2 x + 6 ) e x 6 1 + C . (3x^3+8x^2+2x+6)e^{x^6-1}+C.

Evaluating from 1 -1 to 1 1 gives the final answer:

( 3 ( 1 ) 3 + 8 ( 1 ) 2 + 2 ( 1 ) + 6 ) e 1 6 1 ( 3 ( 1 ) 3 + 8 ( 1 ) 2 + 2 ( 1 ) + 6 ) e ( 1 ) 6 1 (3(1)^3+8(1)^2+2(1)+6)e^{1^6-1}-(3(-1)^3+8(-1)^2+2(-1)+6)e^{(-1)^6-1}

= ( 3 + 8 + 2 + 6 ) ( 3 + 8 2 + 6 ) = 10. =(3+8+2+6)-(-3+8-2+6)=10.

It's always nice to see problems, that Wolfram alpha can't solve. Nice one @James Wilson

Veselin Dimov - 5 months ago

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I just tried the problem in WolframAlpha, and it actually does give an answer of 10.

James Wilson - 5 months ago

Although... it doesn't give a nice antiderivative.

James Wilson - 5 months ago

Thanks! It was inspired by this problem: https://brilliant.org/problems/definite-integral-14/

James Wilson - 5 months ago

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