x^(-27)＋x^(-26)＋…＋x^(-1)＋1＋x＋…＋x^26＋x^27=x^(-27)(1＋x＋x?+x?)+x^(-23)(1＋x＋x?+x?)+…＋x^(-3)(1＋x＋x?+x?)+(1＋x＋x?+x?)＋x^4(1＋x＋x?+x?)＋…＋x^24(1＋x＋x?+x?)-1=-1