Factoring?

Calculus Level 4

PV 0 d x 1 x 3 = a b π c \text{PV} \int_{0}^{\infty} \frac{dx}{1-x^3}= \frac {a\sqrt b \pi}c

The equation above holds true for positive integers a a , b b , and c c with gcd ( a , c ) = 1 \gcd(a,c)=1 and b b being square-free. Find a + b + c a+b+c .

Note:

PV \text{PV} denotes the Cauchy principal value.


The answer is 13.

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2 solutions

Chew-Seong Cheong
Jun 29, 2019

I = 0 d x 1 x 3 Using identity: 0 f ( x ) d x = 0 f ( 1 x ) x 2 d x = 1 2 0 ( 1 1 x 3 + 1 x 2 ( 1 1 x 3 ) ) d x = 1 2 0 ( 1 1 x 3 + x x 3 1 ) d x = 1 2 0 1 x 1 x 3 d x = 1 2 0 1 x 2 + x + 1 d x = 1 2 0 1 ( x + 1 2 ) 2 + 3 4 d x = 2 3 0 1 ( 2 x + 1 3 ) 2 + 1 d x Let u = 2 x + 1 3 d u = 2 3 d x = 1 3 1 3 1 u 2 + 1 d u = 1 3 [ tan 1 u ] 1 3 = 1 3 [ π 2 π 6 ] = π 3 3 = 3 π 9 \begin{aligned} I & = \int_0^\infty \frac {dx}{1-x^3} & \small \color{#3D99F6} \text{Using identity: }\int_0^\infty f(x)\ dx = \int_0^\infty \frac {f\left(\frac 1x\right)}{x^2} dx \\ & = \frac 12 \int_0^\infty \left(\frac 1{1-x^3} + \frac 1{x^2\left(1-\frac 1{x^3}\right)} \right) dx \\ & = \frac 12 \int_0^\infty \left(\frac 1{1-x^3} + \frac x{x^3-1} \right) dx \\ & = \frac 12 \int_0^\infty \frac {1-x}{1-x^3} dx \\ & = \frac 12 \int_0^\infty \frac 1{x^2+x+1} dx \\ & = \frac 12 \int_0^\infty \frac 1{\left(x+\frac 12 \right)^2 + \frac 34} dx \\ & = \frac 23 \int_0^\infty \frac 1{\left(\color{#3D99F6}\frac {2x+1}{\sqrt 3} \right)^2 + 1} dx & \small \color{#3D99F6} \text{Let }u = \frac {2x+1}{\sqrt 3} \implies du = \frac 2{\sqrt 3} dx \\ & = \frac 1{\sqrt 3} \int_{\frac 1{\sqrt 3}}^\infty \frac 1{{\color{#3D99F6}u^2} + 1} du \\ & = \frac 1{\sqrt 3} \left[\tan^{-1} u \right]_{\frac 1{\sqrt 3}}^\infty \\ & = \frac 1{\sqrt 3} \left[\frac \pi 2 - \frac \pi 6\right] \\ & = \frac \pi {3\sqrt 3} = \frac {\sqrt 3 \pi}9 \end{aligned}

Therefore, a + b + c = 1 + 3 + 9 = 13 a+b+c = 1 + 3 + 9 = \boxed{13} .

William Allen
Jun 30, 2019

I = 0 d x 1 x 3 = 0 1 d x 1 x 3 + 1 d x 1 x 3 J I=\displaystyle\int_{0}^{\infty}\frac{dx}{1-x^3}= \displaystyle\int_{0}^{1}\frac{dx}{1-x^3}+\underbrace{\displaystyle\int_{1}^{\infty}\frac{dx}{1-x^3}}_{J}

J = 0 1 x 1 x 3 d x \implies J=\displaystyle\int_{0}^{1}\frac{-x}{1-x^3}dx I = 0 1 1 x 1 x 3 d x = 0 1 d x ( x + 1 2 ) 2 + 3 4 \implies I=\displaystyle\int_{0}^{1}\frac{1-x}{1-x^3}dx = \displaystyle\int_{0}^{1}\frac{dx}{(x+\frac{1}{2})^2+\frac{3}{4}} I = 2 3 arctan ( 3 3 ) 2 3 arctan ( 1 3 ) \implies I=\frac{2}{\sqrt{3}}\arctan({\frac{3}{\sqrt{3}}})-\frac{2}{\sqrt{3}}\arctan({\frac{1}{\sqrt{3}}}) I = 3 π 9 a + b + c = 13 \implies I=\frac{\sqrt{3}\pi}{9} \implies a+b+c=\boxed{13}

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