Q34 of 202 Page 10

If the nth term of an A.P. is 2n + 1, then the sum of first n terms of the A.P. is

a_n= a + (n–1) d = 2n + 1 (given)…………………(1)

S_n= n/2 (2a + (n–1) d)

By putting n=1 in (1)

a=3

similarly a₂= 5

a₃= 7

d= common difference = a₂ – a₁= 2

S_n = n/2 (6 + (n–1) 2)

= n/2(2n + 4)

= n (n + 2)

If , then n =

The sum of n terms of an A.P. is 3n² + 5n, then 164 is its

If 18th and 11th term of an A.P. are in the ratio 3 : 2, then its 21st and 5th terms are in the ratio

The sum of first 20 odd natural numbers is