Question
Thu December 27, 2012 By:
 

If sum of n terms of an AP is 2n^2+5n, then find its nth term?

Expert Reply
Fri December 28, 2012
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
Ask the Expert