Question

Find the sum of first n terms of an A P whose nth term is 3n+1

Answer 1

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Answer 2

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Here is your answer ...

An = 3n+1

put n= 1

A1 = 3 (1)+ 1
= 3+ 1
= 4
===> first term of a.p is 4

put A= 2

A2 = 3(2) + 1
= 6+1
= 7

===> second term of a.p is 7

Common Difference (d ) = A2 - A1
= 7 - 4
= 3
NOW,
Sn = n /2 [2a + ( n-1 ) d ]
= n/ 2 [ 2(4) + (n-1) (3) ]
= n /2 [ 8 + 3n - 3 ]
= n / 2 [ 5 + 3n ]
= 5n + 3n^2/2
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