等差数列{an}中a1+a2+a3=27 a6+a7=36 求an与Sn
问题描述:
等差数列{an}中a1+a2+a3=27 a6+a7=36 求an与Sn
答
a1+a2+a3=27 ,
3a2=27,a2=9,
a6+a7=36,
a2+4d+a2+5d=36,
9d=18,d=2.
an=a2+(n-2)d=9+2(n-2)=2n+5,
a1=7,
Sn=n(a1+an)/2=n(n+6).