Question 26058
 7i^3 + 8i^12 +9i^17 -5i^16 = 
PLEASE NOTE THE FOLLOWING 
I=I
I^2=-1
I^3=I^2*I=-I
I^4=(I^2)^2=(-1)(-1)=1
HENCE IN GENERAL
(I)^EVEN POWER = -1 OR +1..IT IS +1 IF THE POWER IS DIVISIBLE BY 4 OTHER WISE -1

(I)^ODD POWER = I OR -I ....IT IS +I IF THE POWER LESS ONE IS DIVISIBLE BY 4 OTHER WISE -I...
SO NOW YOU CAN DO THE PROBLEM I THINK ..LET US SEE

7i^3 + 8i^12 +9i^17 -5i^16 = -7I+8+9I-5=3-2I...OK...CLEAR?