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Q.

The sum of n terms of an A.P. is 3n2+5n  then its 16th term is,


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a

90

b

94

c

98

d

100 

answer is C.

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Detailed Solution

Given that,
The sum of n terms of an A.P. is 3 n 2 +5n.  
The nth term of an arithmetic progression is given by a n =a+ n1 d  .
The first term is a.
The common difference is d.
The number of terms are n.
For n=1  ,
S 1 =3 1 2 +5 1 S 1 =3+5 S 1 =8   Therefore the first term is, a=8  .
For n=2  ,
  S 2 =3 2 2 +5 2 S 2 =12+10 S 2 =22   Therefore the second term is,
a 2 = S 2 S 1 a 2 =228 a 2 =14   For n=3  ,
S 3 =3 3 2 +5 3 =27+15 =42   Therefore the third term is,
a 3 = S 3 S 2 a 3 =4222 a 3 =20   Thus, the arithmetic progression is, 8,14,20,... .
Here,
a=8 d=6  
Therefore, the 16th term is,
  a 16 =8+ 161 6 a 16 =8+90 a 16 =98  
The 16 th   term is 98.
Hence, option 3 is correct.
 
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