How Complex Is It?

Algebra Level 2

True or False?

( 1 + i ) ( 1 + i ) ( 1 i ) ( 1 i ) = 1 4 ( 1 + 3 i ) ( 1 + 3 i ) ( 1 3 i ) ( 1 3 i ) ( 1 + i)( -1 + i) ( -1-i) (1-i) = \frac{1}{4}( 1 + \sqrt{3}i ) (-1 + \sqrt{3}i ) (-1 - \sqrt{3}i )(1 - \sqrt{3}i )

Clarification : i = 1 i=\sqrt{-1} .

Bonus: Prove it without expanding all the terms.


Inspiration .

True False

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1 solution

Chew-Seong Cheong
Aug 24, 2016

Relevant wiki: De Moivre's Theorem

L H S = ( 1 + i ) ( 1 + i ) ( 1 i ) ( 1 i ) = 2 ( 1 2 + i 2 ) 2 ( 1 2 + i 2 ) 2 ( 1 2 i 2 ) 2 ( 1 2 i 2 ) = 2 e π 4 i 2 e 3 π 4 i 2 e 5 π 4 i 2 e 7 π 4 i = 4 e 4 π i = 4 \begin{aligned} LHS & = (1+i)(-1+i)(-1-i)(1-i) \\ & = \sqrt 2 \left(\frac 1{\sqrt 2} +\frac i{\sqrt 2}\right) \cdot \sqrt 2 \left(-\frac 1{\sqrt 2} +\frac i{\sqrt 2}\right) \cdot \sqrt 2 \left(-\frac 1{\sqrt 2} - \frac i{\sqrt 2}\right) \cdot \sqrt 2 \left(\frac 1{\sqrt 2} - \frac i{\sqrt 2}\right) \\ & = \sqrt 2 e^{\frac \pi 4 i} \cdot \sqrt 2 e^{\frac {3\pi} 4 i} \cdot \sqrt 2 e^{\frac {5\pi} 4 i} \cdot \sqrt 2 e^{\frac {7\pi} 4 i} \\ & = 4 e^{4 \pi i} = 4 \end{aligned}

R H S = 1 4 ( 1 + 3 i ) ( 1 + 3 i ) ( 1 3 i ) ( 1 3 i ) = 1 4 2 ( 1 2 + 3 2 i ) 2 ( 1 2 + 3 2 i ) 2 ( 1 2 3 2 i ) 2 ( 1 2 3 2 i ) = 16 4 e π 3 i e 2 π 3 i e 4 π 3 i e 5 π 3 i = 4 e 4 π i = 4 \begin{aligned} RHS & = \frac 14 (1+\sqrt 3i)(-1+\sqrt 3i)(-1-\sqrt 3i)(1-\sqrt 3i) \\ & = \frac 14 \cdot 2 \left(\frac 12 +\frac {\sqrt 3}2i \right) \cdot 2 \left(-\frac 12 +\frac {\sqrt 3}2i \right) \cdot 2 \left(-\frac 12 -\frac {\sqrt 3}2i \right) \cdot 2 \left(\frac 12 - \frac {\sqrt 3}2i \right) \\ & = \frac {16}4 e^{\frac \pi 3 i} \cdot e^{\frac {2\pi} 3 i} \cdot e^{\frac {4\pi} 3 i} \cdot e^{\frac {5\pi} 3 i} \\ & = 4 e^{4 \pi i} = 4 \end{aligned}

L H S = R H S \implies LHS = RHS , therefore, the answer is True \boxed{\text{True}}

you forgot some of the i i s in the RHS section

Goh Choon Aik - 4 years, 9 months ago

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Thanks. I have inserted them.

Chew-Seong Cheong - 4 years, 9 months ago

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