Typical Omega

Algebra Level 3

( 1 + 3 ) 4 + ( 1 3 ) 4 = ? \large (-1+\sqrt { -3 } )^4+(-1-\sqrt { -3 } )^4= \, ?


The answer is -16.

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2 solutions

Chew-Seong Cheong
Jul 18, 2016

Let a = 1 a=-1 and b = 3 b=\sqrt{-3} . Then we have:

( a + b ) 4 + ( a b ) 4 = a 4 + 4 a 3 b + 6 a 2 b 2 + 4 a b 3 + b 4 + a 4 4 a 3 b + 6 a 2 b 2 4 a b 3 + b 4 = 2 ( a 4 + 6 a 2 b 2 + b 4 ) = 2 ( ( 1 ) 4 + 6 ( 1 ) 2 ( 3 ) 2 + ( 3 ) 4 ) = 2 ( 1 + 6 ( 3 ) + 9 ) = 16 \begin{aligned} (a+b)^4 + (a-b)^4 & = a^4+4a^3b + 6a^2b^2 + 4ab^3 + b^4 + a^4-4a^3b + 6a^2b^2 - 4ab^3 + b^4 \\ & = 2(a^4 + 6a^2b^2 + b^4) \\ & = 2((-1)^4 + 6(-1)^2(\sqrt{-3})^2 + (\sqrt{-3})^4) \\ & = 2(1+6(-3)+9) \\ & = \boxed{-16} \end{aligned}


Alternative Solution

( 1 + 3 ) 4 + ( 1 3 ) 4 = ( 1 + i 3 ) 4 + ( 1 i 3 ) 4 = 2 4 ( 1 2 + i 3 2 ) 4 + 2 4 ( 1 2 i 3 2 ) 4 = 16 ( cos 2 π 3 + i sin 2 π 3 ) 4 + 16 ( cos 2 π 3 + i sin 2 π 3 ) 4 = 16 ( e i 2 π 3 ) 4 + 16 ( e i 2 π 3 ) 4 = 16 e i 8 π 3 + 16 e i 8 π 3 = 16 ( cos 8 π 3 + i sin 8 π 3 ) + 16 ( cos 8 π 3 + i sin 8 π 3 ) = 16 ( cos 8 π 3 + i sin 8 π 3 ) + 16 ( cos 8 π 3 i sin 8 π 3 ) = 32 cos 8 π 3 = 16 \begin{aligned} (-1+\sqrt{-3})^4 + (-1-\sqrt{-3})^4 & = (-1+i\sqrt{3})^4 + (-1-i\sqrt{3})^4 \\ & = 2^4\left(-\frac 12 + i \frac {\sqrt 3}2 \right)^4 + 2^4\left(-\frac 12 - i \frac {\sqrt 3}2 \right)^4 \\ & = 16\left(\cos \frac {2 \pi}3 + i \sin \frac {2 \pi}3 \right)^4 + 16\left(\cos \frac {-2 \pi}3 + i \sin \frac {-2 \pi}3 \right)^4 \\ & = 16\left(e^{i \frac {2 \pi}3} \right)^4 + 16\left(e^{-i \frac {2 \pi}3} \right)^4 \\ & = 16 e^{i \frac {8 \pi}3} + 16 e^{-i \frac {8 \pi}3} \\ & = 16 \left(\cos \frac {8 \pi}3 + i \sin \frac {8 \pi}3 \right) + 16 \left(\cos \frac {-8 \pi}3 + i \sin \frac {-8 \pi}3 \right) \\ & = 16 \left(\cos \frac {8 \pi}3 + i \sin \frac {8 \pi}3 \right) + 16 \left(\cos \frac {8 \pi}3 - i \sin \frac {8 \pi}3 \right) \\ & = 32 \cos \frac {8 \pi}3 \\ & = \boxed{-16} \end{aligned}

Not required since the expression is simply:

16 { e 4 i π / 3 + e 4 i π / 3 } = 32 ( e 4 i π / 3 ) = 32 cos 4 π 3 16\{e^{4i\pi/3}+e^{-4i\pi /3}\}=32\Re(e^{4i\pi/3})=32\cdot \cos \frac{4\pi}3

Rishabh Jain - 4 years, 11 months ago

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Just wanted to show it can be solved using simple algebra.

Chew-Seong Cheong - 4 years, 11 months ago

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Ohk... Great.. (+1)....

Just a genuine query: Where has that option of 'Unsubscribe from comments' gone, previously it used to be in the bottom right corner of our solutions ?

Rishabh Jain - 4 years, 11 months ago

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@Rishabh Jain Don't know, I have never used it but I saw it there.

Chew-Seong Cheong - 4 years, 11 months ago

( 1 + 3 ) 4 + ( 1 3 ) 4 = ( 2. ( 1 + 3 ) 2 ) 4 + ( 2. ( 1 3 ) 2 ) 4 = 2 4 . ω 4 + 2 4 . ( ω 2 ) 4 = 2 4 ( ω + ω 2 ) = 16 × ( 1 ) = 16 { (-1+\sqrt { -3 } ) }^{ 4 }\quad +\quad { (-1-\sqrt { -3 } ) }^{ 4 }=\quad { (2.\frac { (-1+\sqrt { -3 } ) }{ 2 } ) }^{ 4 }\quad +\quad { (2.\frac { (-1-\sqrt { -3 } ) }{ 2 } ) }^{ 4 }\quad \\ ={ 2 }^{ 4 }.{ \omega }^{ 4 }\quad +\quad { 2 }^{ 4 }.({ { \omega }^{ 2 }) }^{ 4 }\\ ={ 2 }^{ 4 }\left( \omega +{ \omega }^{ 2 } \right) \\ =16\quad \times \quad (-1)\quad =\quad \boxed { -16 }

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