(i) The first 15 multiples of 8

a = 8, d = 8

S₁₅= [2(8) = 14(8)]

= 120 [1 + 7]

= 960

(ii) The first 40 positive integers divisible by (a) 3 (b) 5 (c) 6.

(a) a = 3, d = 3

S₄₀= [2(a) = 39(d)]

= 20 [2(3) = 39(3)]

= 60 (41) = 2460

(b) a = 6, d = 5

S₄₀= [2(5) + 39(5)]

= 100 [41]

= 4100

(iii) All 3 – digit natural numbers which are divisible by 13.

a = 6, d = 6

S₄₀= [2(6) + 39(6)]

= 120 (41) = 4920

(iv) All 3 – digit natural numbers, which are multiples of 11.

a = 110, d = 11, l = 990

n = + 1

= + 1

= =

= 81

S₈₁= [2(110) + 80(11)]

= 8910 [1 + 4]

= 44550