A Cute Complex Problem

Question:

If w=\cos{\frac{\pi}{n}}+i\sin{\frac{\pi}{n}}, then find the value of 1+w+w^{2}+w^{3}+\ldots+w^{n-1}.

Solution:

We have S=1+w+w^{2}+w^{3}+\ldots+w^{n-1}=\frac{1-w^{n}}{1-w}.

But, w^{n}=\cos{\frac{n\pi}{n}}+i\sin{\frac{n\pi}{n}}=-1

Thus, S=\frac{2}{1-w}

but, 1-w=1-\cos{\frac{\pi}{n}}-i\sin{\frac{\pi}{n}} which equals

2\sin^{2}{\frac{\pi}{2n}}-2i\sin{\frac{\pi}{2n}}\cos{\frac{\pi}{2n}}

that is, -2i\sin{\frac{\pi}{2n}}[\cos{\frac{\pi}{2n}}+i\sin{\frac{\pi}{2n}}].

Thus, S=\frac{-2}{2i\sin{\frac{\pi}{2n}}}[\cos{\frac{\pi}{2n}}+i\sin{\frac{\pi}{2n}} ]^{-1}=1+i\cot{\frac{\pi}{2n}}.

Hope you are finding it useful,

More later,

Nalin Pithwa

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