1+x+x²+x³=(1-x^4)/(1-x)=0
——》1-x^4=0,1-x≠0,
——》x=-1,
1＋x＋x²＋x³＋﹍＋x²ºº⁴
=(1-x^2005)/(1-x)
=[1-(-1)^2005]/[1-(-1)]
=(1+1)/(1+1)
=1.