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KEAM 2015 Math Paper
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© examsnet.com
Question : 41
Total: 120
An A.P. consists of 23 terms. If the sum of the 3 terms in the middle is 141 and the sum of the last 3 terms is 261, then the first term is
6
5
4
3
2
Validate
Solution:
Let the first term and common difference of AP be a and
d
, respectively.
Then,
a
11
+
a
12
+
a
13
=
141
⇒
3
a
+
33
d
=
141
⇒
a
+
11
d
=
47
....(i)
Also,
a
21
+
a
22
+
a
23
=
261
⇒
3
a
+
63
d
=
261
⇒
a
+
21
d
=
87
.....(ii)
From Eqs. (i) and (ii),
a
=
3
© examsnet.com
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