Question

The sum of first n terms of an ap is 3n^2-4n, find its 25th term

Answer 1

Step-by-step explanation:

Sn=3n²-4n

n/2{2a+(n-1)d}=n(3n-4)

1/2{2a+(n-1)d}=(3n-4)

2a+(n-1)d=6n-8

2a+nd-d=6n-8

(2a-d)+nd=6n-8

by seperating

nd=6n & (2a-d)= -8

d=6 & 2a-6= -8

a= -1

tn=a+(n-1)d

t25=a+(25-2)d

= -1+24×6

= -1+144

=143