由已知条件得到：a6=a1+(5-1)d=a1+4d=-5s4=4a1+(4-1)*4/2*d=4a1+6d=-64联立解得：a1=-145/7 d=22/7an=a1+(n-1)d=-145/7+22/7*(n-1)令an≥0 得到n≥167/22 即n≥8∴当n＜8时 an＜0；当n≥8时,an ＞0S7=7a1+(7-1)*7/2*...