Answer: -430
Step-by-step explanation: The formula for the sum of an arithmetic sequence when
a
,
d
and
n
are given is:
S
u
m
=
(
n
2
)
[
2
a
+
(
n
−
1
)
d
]
Here :
a
1
=
7
,
d
=
−
3
and
n
=
20
=
(
20
2
)
[
2
.
7
+
(
20
−
1
)
.
−
3
]
=
10
.
[
14
+
19
.
(
−
3
)
]
=
10
.
[
14
−
57
]
=
10
.
(
−
43
)
=
−
430