(1)
Sn=(a1+an)n/2
(20+54)n/2=999
n=27
an=a1+(n-1)d
d=(an-a1)/(n-1)=(54-20)/(27-1)=17/13
(2)
估计题目抄错了,求an,可an已经给了,估计是求Sn
an=a1+(n-1)d
n=(an-a1)/d +1=(-5-5/6)/(-1/6) +1=36
Sn=(a1+an)n/2=(5/6 -5)×36/2=-75第⑵的an=-5，是Sn=-5，我抄错了。。。。。哦，那么：Sn=na1+n(n-1)d/2(5/6)n+(-1/6)n(n-1)/2=-5整理，得n²-11n-60=0(n+4)(n-15)=0n=-4(舍去)或n=15an=a1+(n-1)d=5/6+(-1/6)(15-1)=-3/2n=15an=-3/2