(Q15).Find the sum of :
(iv)
1
15
,
1
12
,
1
10
,…, to 11 terms.
Given A.P:
1
15
,
1
12
,
1
10
,…, S
11
.
a =
1
; d = a
2
- a
1 =
1
-
1
=
-
=
; n =
S
n
=
n
2
[2a + (n – 1)d]
S
11
=
2
[2 ×
1
(
-1) ×
1
]
=
2
[
+
]
=
2
[
× 2 +
× 1
]
=
2
[
+
]
=
2
×
=
=
Clear