The answer is an integer? Impossible!

Calculus Level 4

What is the value of

7 ( π + 0 1 x 4 ( 1 x ) 4 1 + x 2 d x ) ? 7 \left(\pi + \int_0^1 \dfrac{x^4(1-x)^4}{1+x^2} \, dx\right) ?


The answer is 22.

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4 solutions

Chew-Seong Cheong
Aug 28, 2019

I = 0 1 x 4 ( 1 x ) 4 1 + x 2 d x = 0 1 x 8 4 x 7 + 6 x 6 4 x 5 + x 4 x 2 + 1 d x = 0 1 x 6 4 x 5 + 5 x 4 4 x 2 + 4 x 2 + 1 d x = 1 7 2 3 + 1 4 3 + 4 4 tan 1 1 = 22 7 π \begin{aligned} I & = \int_0^1 \frac {x^4(1-x)^4}{1+x^2} dx \\ & = \int_0^1 \frac {x^8-4x^7+6x^6-4x^5+x^4}{x^2+1} dx \\ & = \int_0^1 x^6 - 4x^5 + 5x^4 - 4x^2 + \frac 4{x^2+1} dx \\ & = \frac 17 - \frac 23 + 1 - \frac 43 + 4 - 4\tan^{-1} 1 \\ & = \frac {22}7- \pi \end{aligned}

Therefore, 7 ( π I ) = 22 7(\pi - I) = \boxed {22} .


Note: Since I > 0 I > 0 , 22 7 > π \implies \frac {22}7 > \pi . In fact the integral is used to show that 22 7 \frac {22}7 is only an approximation of π \pi .

Jeremiah Jocson
Mar 20, 2015

use partial fraction techniques, then evaluate the problem

hence the answer 7[(10/3)+(1/105)-(1/5)] = 22

Incredible Mind
Feb 11, 2015

just expand the (1-x^4)...divide by x^4..then indefinite integrall.. and finally plug in the limits..

definite integral becomes 22/7 - pi

hence ANS is 22

Is there any other method to do this?

Prakash Chandra Rai - 6 years, 4 months ago

@incredible mind typo in the first line of the solution. @Calvin Lin

Ankit Kumar Jain - 3 years ago
Lu Chee Ket
Feb 11, 2015

7 (0.00126448926734962 + 3.14159265358979) =

7 (3.14285714285714000) = 22

Unbelievable.

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