i=i
i^2=-1 (stim din definitie)
i^3=i^2 *i=-i
i^4=(i^2)^2=1
i+i^2+i^3+i^4=i-1-i+1=0
la fel e si pt i^5+i^6+i^7+i^8=0
o numim pereche si are 4 termeni
deci 2001/4=500, rest 1
sunt 500 de astfel de perechi care dau 0 si inca un numar, i^2001
i^2001=i* (i^2)^1000=i* (-1)^1000
(-1)^1000=1 =>i^2001=i
deci
i+i^2+i^3+....+i^2001=0+i=i
