Question

The sum of the first n terms of an ap is 4n2 -3n find the ap abd hence find 12th teekm

Answer 1

Given that sum of the first n terms of an ap is $S_n = 4n^2 -3n$

When we substitute 1 for n, we get sum of 1 term = I term.

When we substitute 2 for n, we get sum of 2 terms = a1+a2

From this we get a2.

Since a1 = S1 we get

first term = 4(1)-3 = 1

S2 = 4(4)-3(2) = 10

a2 = S2-S1 = 10-1 =9

d = a2-a1 = 9-1 =8

Arithmetic progression is

1, 9, 17, 25,......

12th term = a12 = a+11d= 1+88 = 89