Question

Sn=3n2-5n find nth term and find a20

Answer 1

Find an

Given : Sn = 3n^2 - 5n

Now find out S(n - 1)
= 3(n - 1)^2 - 5(n - 1)
= 3 [n^2 + 1 - 2n] - 5n + 5
= 3n^2 + 3 - 6n - 5n + 5
= 3n^2 - 11n + 8

Now subtract S(n - 1) from Sn to get an
an = Sn - S(n-1)
= (3n^2 - 5n) - (3n^2 - 11n + 8)
= (3n^2 - 5n - 3n^2 + 11n - 8)
an = 6n - 8

Now find
a20 = (6 x 20) - 8
a20 = 120 - 8
a20 = 108