Question

ih nth term of an AP is 1/3 (2n+1), then the sum of its 19 terms is

Answer 1

an=1/3(2n+1)
a1=1/3(2×1+1)
=1/3×3
=1
a2=1/3(2×2+1)
5/3
a19=1/3(2×19+1)
=1/3(38+1)
=1/3×39
=13
s19=19/2(a+a19)
=19/2(1+13)
=19/2×14
=19×7
=133