(Q16). Find the sums given below.
(i) 7 + 10
1
2
+ 14 + …. + 84
Given A.P : 7 + 10
1
2
+ 14 + …. + 84
a =
; d = a
2
– a
1
=
1
–
=
1
and the last term l = a
n
=
But, a
n
= a + (n – 1) d
∴
=
+ (n – 1)
1
⇒
–
= (n – 1) ×
⇒ n – 1 =
×
=
⇒ n =
+ 1 =
Now, S
n
=
n
2
(a + l) where a =
; l =
S
23
=
2
(
+
)
=
2
×
=
2
=
1
Clear