等差数列{an}中,公差为d,已知a4=84,前n项和为Sn,且S10>0,S11
问题描述：
等差数列{an}中,公差为d,已知a4=84,前n项和为Sn,且S10>0,S11
答
1.
A4=A1+3d=84
A1=84-3d
Sn=nA1+(1/2)n(n-1)d
=n(84-3d)+(1/2)n(n-1)d
=84n+(1/2)n(n-7)d
S10=840+15d>0d>-56
S11=924+22d