A Very Odd Integral

Calculus Level 3

π / 4 π / 4 x 3 sec 2 x d x \large \int _{ -{ \pi }/{ 4 } }^{ { \pi }/{ 4 } }{ \, x^3 \sec^2{x} }~ {dx}

If the integral above is equal to π A B \pi^A -B , where A A and B B are integers, find A + B A+B .


The answer is 1.

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1 solution

Michael Fuller
Feb 16, 2016

Let f ( x ) = x 3 sec 2 x f(x)=x^3 \sec^2 x . Then f ( x ) = ( x ) 3 sec 2 ( x ) = ( x 3 sec 2 x ) = f ( x ) f(-x)={(-x)}^{3} \sec^2 {(-x)} = -(x^3 \sec^2 x) = -f(x) . Therefore the function is odd and since the function is defined for x [ π 4 , π 4 ] x \in [ -\frac{\pi}{4}, \, \frac{\pi}{4} ] the integral is equal to 0 0 .

We need to find two integers A A and B B such that π A B = 0 {\pi}^{A}-{B}=0 , and the only way for this to be possible is if A = 0 A=0 (any other integer will include pi), and therefore B = 1 B=1 .

Therefore A + B = 0 + 1 = 1 A+B=0+1=\large \color{#20A900}{\boxed{1}} .

Its also written in title ;)

harish ghunawat - 5 years, 3 months ago

Oh no , I knew the answer was 0. But I was tricked by π A B \pi^{A}-B and hence clicked discuss solutions :P

Nihar Mahajan - 5 years, 3 months ago

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Same story here.... It's too confusing .. I also got the correct answer in 3rd attempt ;-) ..

Rishabh Jain - 5 years, 3 months ago

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