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What is the smallest positive integer value of  n such that (√3/2-i/2)^n is real?

 Mar 17, 2024
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Consider the expression (3​/2−i/2)n. Let's analyze the powers of this complex number according to the four cases for the remainder r of n when divided by 4:

 

Case r=0: Then n=4k for some integer k, and

 

[(\sqrt{3}/2 - i/2)^{4k} = \left( (\sqrt{3}/2)^2 + (i/2)^2 \right)^{2k} = \left( \frac{3}{4} + \frac{1}{4} \right)^{2k} = 1^{2k} = 1.]

 

Since 1 is real, this case produces a real number.

 

Case r=1: Then n=4k+1 for some integer k, and

 

(3/2i/2)4k+1=(3/2i/2)(3/2i/2)4k =(3/2i/2)1 =3/2i/2.

 

Since this expression has a nonzero imaginary part, it is not real.

 

Case r=2: Then n=4k+2 for some integer k, and

 

(3/2i/2)4k+2=(3/2i/2)2(3/2i/2)4k =(34234i14)1 =34234i.

 

Since this expression has a nonzero imaginary part, it is not real.

 

Case r=3: Then n=4k+3 for some integer k, and

 

(3/2i/2)4k+3=(3/2i/2)(3/2i/2)4k+2 =(3/2i/2)(34234i) =334+1434i.

 

Since this expression has a nonzero imaginary part, it is not real.

 

In conclusion, the smallest positive integer value of n such that (3​/2−i/2)^n is real is 4​.

 Mar 17, 2024

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