Complex but easy

When $i$ is raised to an even power, the result is $-1$ or $1$ depending on if the power is divisible by $4.$ In this case, the power is not divisible by $4,$ since $78$ is not divisible by $4.$ When the even power is not divisible by $4,$ the result is $\boxed{-1}.$

we know i^4=1 so i^(4n+m)=i^(4n)×i^m=1×i^m=i^m Now i^(012345678)=i^(4×012345676+2)=i^2=-1