题目

计算
(1){an}为等差数列且a1=20,an=54,sn=999,求d及n
{1)已知等比数列a1=-1,a4=64a求q及s
{2)已知等比数列a1=-1，a4=64，求q与s4 回答这个第二题，上面的第二题是错的

提问时间：2020-10-09

答案

1.an =a1+(n-1)d= 20+(n-1)d=54(n-1)d=34Sn =na1+ nd(n-1)/2 =20n+34n/2=37n=999n = 27d = 34/(27-1) = 17/132.a4 = a1*q^364 = -q^3q = -4s4 = a1(1-q^4)/(1-q) = [(-4)^4 -1]/5 = 51