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

1.an =a1+(n-1)d= 20+(n-1)d=54
(n-1)d=34
Sn =na1+ nd(n-1)/2 =20n+34n/2=37n=999
n = 27
d = 34/(27-1) = 17/13
2.a4 = a1*q^3
64 = -q^3
q = -4
s4 = a1(1-q^4)/(1-q) = [(-4)^4 -1]/5 = 51