已知数列{1/an+2}成等差数列,且a3=-11/6,a5=-13/7,则a8=_.
问题描述:
已知数列{
}成等差数列,且a3=-1
an+2
,a5=-11 6
,则a8=______. 13 7
答
∵数列{
}成等差数列,且a3=-1
an+2
,a5=-11 6
,13 7
∴d=
(1 2
−1
a5+2
)1
a3+2
=
(1 2
−1 −
+213 7
)1 −
+211 6
=
,1 2
∵
+2×1
a1+2
=1 2
,1 −
+211 6
∴
=5,1
a1+2
∴
=5+7×1
a8+2
=1 2
,17 2
解得a8=-
.32 17
故答案为:-
.32 17