It needs some construction

Algebra Level 3

Find the value of ( i + 3 ) 100 + ( i 3 ) 100 + 2 100 , (i+\sqrt{3})^{100}+(i-\sqrt{3})^{100}+2^{100} , where i = 1 i=\sqrt{-1} .

1 -1 2 0

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5 solutions

Sanjeet Raria
Oct 31, 2014

A complex number a + i b a+ib for which a : b = 1 : 3 o r 3 : 1 |a:b|=1:√3\space or \space √3:1 can always be put in terms of ω \omega (the second cube root of unity).

Now ( i + 3 ) = 2 i ω (i+√3)=-2i\omega ( i 3 ) = 2 i ω 2 (i-√3)=-2i\omega^2 (Try proving them)

So that our required expression becomes, 2 100 ω + 2 100 ω 2 + 2 100 2^{100}\omega+ 2^{100}\omega^2+ 2^{100} = 2 100 ( ω + ω 2 ) + 2 100 =2^{100}(\omega+\omega^2)+ 2^{100} = 2 100 ( 1 ) + 2 100 = 0 = 2^{100}(-1)+ 2^{100}=\boxed 0

x 3 1 = 0 x^{3} -1 = 0

( x 1 ) ( x 2 + x + 1 ) = 0 (x - 1)(x^{2} + x +1) = 0

thus x = 1 , 1 + i 3 2 , 1 i 3 2 x = 1 , \frac{ -1 + i\sqrt{3}}{2} , \frac{ -1 - i\sqrt{3}}{2}

w = 1 + i 3 2 , w 2 = 1 i 3 2 w = \frac{ -1 + i\sqrt{3}}{2} , w^{2} = \frac{ -1 - i\sqrt{3}}{2}

2 i w = i + i 2 3 = 2 i w 2iw = -i + i^{2}\sqrt{3} = -2iw

2 i w 2 = i i 2 3 = 2 i w 2 2iw^{2} = -i - i^{2}\sqrt{3} = -2iw^{2}

Nice observation @Sanjeet Raria

U Z - 6 years, 7 months ago
Chew-Seong Cheong
Oct 31, 2014

We can use the Euler's identity: e i θ = cos θ + i sin θ e^{i\theta} = \cos {\theta} + i\sin{\theta}

( i + 3 ) 100 + ( i 3 ) 100 + 20 0 100 (i+\sqrt{3})^{100} + (i-\sqrt{3})^{100} +200^{100}

= 2 100 ( 3 2 + i 1 2 ) 100 + 2 100 ( 3 2 + i 1 2 ) 100 + 20 0 100 =2^{100}(\frac {\sqrt{3}}{2}+i\frac {1}{2} )^{100} + 2^{100}(\frac {-\sqrt{3}}{2}+i\frac {1}{2} )^{100} +200^{100}

= 2 100 [ ( cos π 6 + i sin π 6 ) 100 + ( cos 5 π 6 + i sin 5 π 6 ) 100 + 1 ] =2^{100} \left[ (\cos{\frac {\pi}{6} }+i \sin {\frac {\pi}{6} })^{100} + (\cos{\frac {5\pi}{6} }+i \sin {\frac {5\pi}{6} } )^{100} + 1 \right]

= 2 100 [ ( e i π 6 ) 100 + ( e i 5 π 6 ) 100 + 1 ] = 2 100 ( e i 100 π 6 + e i 500 π 6 + 1 ) =2^{100} \left[ (e^{i \frac {\pi}{6}}) ^{100} + (e^{i\frac {5\pi}{6}}) ^{100} + 1 \right] = 2^{100} (e^{i \frac {100\pi}{6}} + e^{i\frac {500\pi}{6}} + 1 )

= 2 100 ( e i 16 2 π 3 + e i 83 π 3 + 1 ) = 2 100 ( e i 2 π 3 + e i 4 π 3 + 1 ) =2^{100} (e^{i16 \frac {2\pi}{3}} + e^{i83\frac {\pi}{3}} + 1 ) =2^{100} (e^{i \frac {2\pi}{3}} + e^{i\frac {4\pi}{3}} + 1 )

= 2 100 ( cos 2 π 3 + i sin 2 π 3 + cos 4 π 3 + i sin 4 π 3 + 1 ) =2^{100} (\cos{\frac {2\pi}{3} }+i \sin {\frac {2\pi}{3} } + \cos{\frac {4\pi}{3} }+i \sin {\frac {4\pi}{3} } + 1 )

= 2 100 ( 1 2 + i 3 2 1 2 i 3 2 + 1 ) = 0 =2^{100}( -\frac {1}{2} + i\frac {\sqrt{3}}{2} - \frac {1}{2} - i\frac {\sqrt{3}}{2}+1 ) = \boxed {0}

Renah Bernat
Nov 4, 2014

Using De Moivre's Thm: (3^0.5 + i)^100 + (3^0.5 - i)^100 +2^100 = 2^100(cis(100pi/6)) - 2^100(cis(-100pi/6)) + 2^100 = 2^100(-0.5 + (3^0.5)(i/2)) - 2^100(0.5 - (3^0.5)(i/2)) + 2^100 = 2^100 - 2^100 = 0

Marcus Gustafsson
Oct 31, 2014

I used de Moivre after rewriting in polar form. Then it was straightforward, using the periodicity of trigonometric functions and some factorization, but not as elegant as the previous solution.

Niaz Ghumro
Nov 4, 2014

(-2iw)^100+(-2iw^2)^2+2^100=(2w)^100+(2w^2)^100+2^100=2^100(w+w^2+1)=2^100(0)=0

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