z^2= a2+2ai-1;
z^3=a^3+2(a^2)i-a + (a^2)i-2a-i
=(a^3-3a) +(3a^2-1)i
因为是纯虚数,
a^3-3a=0;
3a^2-1 ≠0
解得 a=0或正负根号三