[tex]\bf 2i^{15}-i^7\qquad
\begin{cases}
i^2=\sqrt{-1}\cdot \sqrt{-1}\implies \sqrt{(-1)^2}\implies -1\\\\
i^4=i^2\cdot i^2\implies -1\cdot -1\implies 1
\end{cases}\\\\
-----------------------------\\\\
2i^{4+4+4+2+1}-i^{4+2+1}\implies 2i^4i^4i^4i^2i^1-i^4i^2i^1
\\\\\\
(2\cdot 1\cdot 1\cdot 1\cdot -1\cdot i)-(1\cdot -1\cdot i)\implies -2i-(-i)
\\\\\\
-2i+i\implies -i\implies \boxed{0-i}[/tex]
