Question

高一数学题求等差数列{an}的有关未知数

Answer 1

(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