X后面的数字是指数吧?从后面开始,五个数一组,可得：原式＝(1＋x^1＋x^2＋x^3＋x^4)＋(x^5＋x^6＋x^7＋x^8＋x^9)＋…＋(x^1995＋x^1996＋x^1997＋x^1998＋x^1999)＝(1＋x^1＋x^2＋x^3＋x^4)＋x^5(1＋x^1＋x^2＋x^3＋x...