# An algebra problem by Abu Bakar Khan

Algebra Level 3

$\large \sum_{r=1}^{25} i^{r!} = \, ?$

Notations:

• $$i = \sqrt {-1}$$ is the imaginary unit.
• $$!$$ is the factorial notation. For example, $$8! = 1\times2\times3\times\cdots\times8$$.
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