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Byju's Answer
Standard X
Mathematics
Patterns, Sequences
Question 6 Th...
Question
Question 6
The
21
s
t
term of an AP whose first
two terms are – 3
and 4, is
A) 17
B) 137
C) 143
D) -143
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Solution
Given, the first two terms of the AP are :
a = -3 and
a + d = 4.
⇒
−
3
+
d
=
4
⇒
d
=
7
∴
a
21
=
a
+
(
21
−
1
)
d
[
∵
a
n
=
a
+
(
n
−
1
)
d
]
⇒
a
21
=
−
3
+
(
20
)
7
⇒
a
21
=
−
3
+
140
=
137
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