sum of n terms of an ap is 3n^2+5n . find the ap. find it's 15th term.
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The sum of n terms of an A.P. = 3n2+5n
then sum of n-1 terms= 3(n-1)2+5(n-1)
So nth term of A.P will be Tn=3n2+5n- 3(n-1)2-5(n-1)= 3n2+5n-3n2-3+6n-5n+5=6n+2
in this way by putinng values of n= 1,2,3,4,5,6,7,_______n-1, n
we will get the AP as 8,14,20,26,32,38,44.......
and 15th term =a+(n-1)d=8+(15-1)6=8+112=120 ans.
then sum of n-1 terms= 3(n-1)2+5(n-1)
So nth term of A.P will be Tn=3n2+5n- 3(n-1)2-5(n-1)= 3n2+5n-3n2-3+6n-5n+5=6n+2
in this way by putinng values of n= 1,2,3,4,5,6,7,_______n-1, n
we will get the AP as 8,14,20,26,32,38,44.......
and 15th term =a+(n-1)d=8+(15-1)6=8+112=120 ans.
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