Question

sum of first n terms of an ap whose nth term
is 3 n + 1

Answer 1

an = 3n + 1
for n=1, a1 = 3×1 + 1 = 4
for n =2, a2 = 3 ×2 +1 = 7
d = a2 - a1 = 7 -4 = 3

sum of first n terms = n/2 { 2a + (n-1)d}
Sn = n/2 { 2× 4 + (n -1)×3}
Sn = n/2 { 8 + 3n - 3}
Sn = n/2 { 5 + 3n }