Find the value of ( i + 3 ) 1 0 0 + ( i − 3 ) 1 0 0 + 2 1 0 0 , where i = − 1 .
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x 3 − 1 = 0
( x − 1 ) ( x 2 + x + 1 ) = 0
thus x = 1 , 2 − 1 + i 3 , 2 − 1 − i 3
w = 2 − 1 + i 3 , w 2 = 2 − 1 − i 3
2 i w = − i + i 2 3 = − 2 i w
2 i w 2 = − i − i 2 3 = − 2 i w 2
Nice observation @Sanjeet Raria
We can use the Euler's identity: e i θ = cos θ + i sin θ
( i + 3 ) 1 0 0 + ( i − 3 ) 1 0 0 + 2 0 0 1 0 0
= 2 1 0 0 ( 2 3 + i 2 1 ) 1 0 0 + 2 1 0 0 ( 2 − 3 + i 2 1 ) 1 0 0 + 2 0 0 1 0 0
= 2 1 0 0 [ ( cos 6 π + i sin 6 π ) 1 0 0 + ( cos 6 5 π + i sin 6 5 π ) 1 0 0 + 1 ]
= 2 1 0 0 [ ( e i 6 π ) 1 0 0 + ( e i 6 5 π ) 1 0 0 + 1 ] = 2 1 0 0 ( e i 6 1 0 0 π + e i 6 5 0 0 π + 1 )
= 2 1 0 0 ( e i 1 6 3 2 π + e i 8 3 3 π + 1 ) = 2 1 0 0 ( e i 3 2 π + e i 3 4 π + 1 )
= 2 1 0 0 ( cos 3 2 π + i sin 3 2 π + cos 3 4 π + i sin 3 4 π + 1 )
= 2 1 0 0 ( − 2 1 + i 2 3 − 2 1 − i 2 3 + 1 ) = 0
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
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.
(-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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A complex number a + i b for which ∣ a : b ∣ = 1 : √ 3 o r √ 3 : 1 can always be put in terms of ω (the second cube root of unity).
Now ( i + √ 3 ) = − 2 i ω ( i − √ 3 ) = − 2 i ω 2 (Try proving them)
So that our required expression becomes, 2 1 0 0 ω + 2 1 0 0 ω 2 + 2 1 0 0 = 2 1 0 0 ( ω + ω 2 ) + 2 1 0 0 = 2 1 0 0 ( − 1 ) + 2 1 0 0 = 0