Answer
$-i$
Work Step by Step
$i^{0}=1$
$i^{1}=i$
$i^{2}=-1$
$i^{3}=-i$
$i^{4}=1 ...$
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$i^{43}=i^{40+3}=$
$=i^{40}\cdot i^{3}$
$=(i^{4})^{10}\cdot i^{3}$
$=1^{10}\cdot(-i)$
$=-i$
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