Question

If the sum of first n terms of an A.P. is 2n2 + 5n, then find its nth term.
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Answer 1

Answer:

Sum of first n terms of AP, Sn = 2n2 + 5n

 

Now choose n =1 and put in the above formula,

First term = 2+5 = 7

 

Now put n=2 to get the sum of first two terms = 2x4 + 5x 2= 8 + 10 = 18

 

This means  

 

first term + second term = 18

 

but first term =7 as calculated above

 

so, second term = 18-7 = 11

 

So common difference becomes, 11-7 = 4

 

So the AP becomes, 7, 11, 15, ....

nth term = a + (n - 1)d = 7 + (n-1)4 = 7 + 4n - 4 = 3 + 4n

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Step-by-step explanation: