Unification

Algebra Level 5

e i π / 8 + e 3 i π / 8 = α + β γ ( a + b i ) \large { e }^{ { i\pi }/{ 8 } }+{ e }^{ { 3i\pi }/{ 8 } }=\sqrt { \dfrac { \alpha +\sqrt { \beta } }{ \gamma } } (a+bi)

The equation above holds true for positive integers α , β , γ , a , \alpha ,\beta ,\gamma ,a, and b b , with α , β , \alpha ,\beta , and γ \gamma square-free.

Find α a × β b × γ { \alpha }^{ a }\times { \beta }^{ b }\times \gamma .

Clarifications :

  • e e denotes Euler's number , e 2.71828 e \approx 2.71828 .

  • i = 1 i=\sqrt{-1} .


The answer is 8.

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1 solution

Alex G
Jun 9, 2016

Factoring out e i π 4 \large e^{i\dfrac{\pi}{4}} :

e i π 4 ( e i π 8 + e i π 8 ) \LARGE e^{i \frac {\pi} {4} } \left( e^{-i \frac {\pi} {8} }+e^{ i\frac{\pi} {8} } \right)

Now this problem is purely computational. Using Euler's Formula, the question becomes

( cos ( π 4 ) + i sin ( π 4 ) ) ( cos ( π 8 ) + cos ( π 8 ) + i ( sin ( π 8 ) + sin ( π 8 ) ) ) \left(\cos{\left(\dfrac{\pi}{4}\right)}+ i \sin{\left(\dfrac{\pi}{4}\right)} \right) \left(\cos{\left(-\dfrac{\pi}{8}\right)} + \cos{\left(\dfrac{\pi}{8}\right)} + i \left( \sin{\left(-\dfrac{\pi}{8}\right)} + \sin{\left(\dfrac{\pi}{8}\right)} \right) \right)

Using the facts that sine is an odd function and cosine is even function:

( cos ( π 4 ) + i sin ( π 4 ) ) ( 2 cos ( π 8 ) ) \left(\cos{\left(\dfrac{\pi}{4}\right)}+ i \sin{\left(\dfrac{\pi}{4}\right)} \right) \left(2\cos{\left(\dfrac{\pi}{8}\right)} \right)

Compute cos ( π 8 ) \cos{\left(\dfrac{\pi}{8}\right)} using the cos ( x 2 ) = 1 + cos x 2 \cos{\left(\dfrac{x}{2}\right)} = \sqrt{\dfrac{1+\cos x}{2}} , an identity that can be derived from the power reducing formula. Evaluating cos ( π 8 ) \cos{\left(\dfrac{\pi}{8}\right)} and cos ( π 4 ) \cos{\left(\dfrac{\pi}{4}\right)}

( 2 + 2 ) ( 1 2 + i 1 2 ) \left(\sqrt{2+\sqrt{2}}\right)\left(\dfrac{1}{\sqrt 2} + i \dfrac{1}{\sqrt 2} \right)

Factoring out 1 2 \dfrac{1}{\sqrt 2} :

( 2 + 2 2 ) ( 1 + i ) \left(\sqrt{\dfrac{2+\sqrt{2}}{2}}\right)\left(1 + i \right)

This is the form asked for by the problem. Evaluating α a × β b × γ { \alpha }^{ a }\times { \beta }^{ b }\times \gamma , the answer is

8 \boxed {8}

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