{an}为等差数列,a1003+a1004+a1005+a1006=18,则s2008=?
问题描述:
{an}为等差数列,a1003+a1004+a1005+a1006=18,则s2008=?
答
a1003+a1004+a1005+a1006=18
(a1003+a1006)+(a1004+a1005)=18
2(a1003+a1006)=18
a1+a2008=a1003+a1006=9
s2008=2008(a1+a2008)/2=1004*9=9036