Question
Find the value of $1+i^2+i^4+i^6+i^8+\ldots \ldots+i^{20}$.

Answer

$\begin{aligned} & 1+i^2+i^4+i^6+i^8+\ldots .+i^{20} \\ & =1+\left(i^2+i^4\right)+\left(i^6+i^8\right)+\left(i^{10}+i^{12}\right)+\left(i^{14}+i^{16}\right)+\left(i^{18}+i^{20}\right) \\ & =1+\left[i^2+\left(i^2\right)^2\right]+\left[\left(i^2\right)^3+\left(i^2\right)^4\right]+\left[\left(i^2\right)^5+\left(i^2\right)^6\right]+\left[\left(i^2\right)^7+\left(i^2\right)^8\right]+\left[\left(i^2\right)^9+\left(i^2\right)^{10}\right] \\ & =1+\left[-1+(-1)^2\right]+\left[(-1)^3+(-1)^4\right]+\left[(-1)^5+(-1)^6\right]+\left[(-1)^7+(-1)^8\right]+\left[(-1)^9+(-1)^{10}\right]\left[\because i^2=\right. \\ & -1] \\ & =1+(-1+1)+(-1+1)+(-1+1)+(-1+1)+(-1+1) \\ & =1+0+0+0+0+0 \\ & =1\end{aligned}$

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