设{an}为等差数列则（1）d=-2,a16=1,求a2,s10 (2)d=1/3,n=37,sn=629求a1,an (3)a6=5,a3+a8=5,求s9

题目

设{an}为等差数列则（1）d=-2,a16=1,求a2,s10 (2)d=1/3,n=37,sn=629求a1,an (3)a6=5,a3+a8=5,求s9
(4)sn=5n²+3n,求a1,a2,a3及an

答案

(1)a16=a1+15d a1=31
a2=29 S10=a1n+n(n-1)d/2=220
(2)S37=629=37a1+37*36*(1/3)/2
a1=11
an=11+1/3(n-1)=n/3+32/3
(3)a3+a8=5=a6+a5
a5=0
S9=(a1+a9)9/2=2a5*9/2=0
(4) a1=s1=8
a2=18
a3=28
an=Sn-Sn-1=10n-2(n属于N+)