hint: [tex]\bf \begin{array}{llll}
i^{3}\to & i^2\cdot i^1\\\\
i^{4}\to & i^2\cdot i^2\\\\
i^{5}\to & i^2\cdot i^2\cdot i^1\\\\
i^{6}\to & i^2\cdot i^2\cdot i^2\\\\
i^{7}\to & i^2\cdot i^2\cdot i^2\cdot i^1\\\\
i^{4n}\to & (i^{4})^n\\\\
i^{4n+1}\to & (i^4)^n\cdot i^1\\\\
i^{4n+2}\to & (i^4)^n\cdot i^2\\\\
i^{4n+3}\to & (i^4)^n\cdot i^3
\end{array}[/tex]
