(Q20).In an AP:
(ii) Given a = 7, a
13
= 35, find d and S
13
.
Given : a =
;
a
13
= a +
d =
⇒
+
d =
⇒
d =
–
⇒ n =
=
Now, S
n
=
n
2
(a + l)
S
13
=
2
(
+
)
=
2
(
)
=
×
=
Clear