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{

1
an+2
}成等差数列,且a3=-
11
6
,a5=-
13
7
,则a8=______.

∵数列{

1
an+2
}成等差数列,且a3=-
11
6
,a5=-
13
7

∴d=
1
2
(
1
a5+2
1
a3+2
)

=
1
2
(
1
13
7
+2
1
11
6
+2
)

=
1
2

1
a1+2
+2×
1
2
=
1
11
6
+2

1
a1+2
=5,
1
a8+2
=5+7×
1
2
=
17
2

解得a8=-
32
17

故答案为:-
32
17