Question

the sum of all natural numbers between 100 to 200 which are multiples of 3

Answer 1

the answer is 4950 hope this help

Answer 2

102 is the first number after 100 which is divisible by 3 . So, a = 102.
d = 3.
198 is the last term before 200 which is divisible by 3. So, an = 198.
an = a+(n-1)d = 198
(n-1)d = 198-102
n-1 = 96/3
n = 32+1
n = 33.
Sn = n/2(a+an)
Sn = 33/2(102+198)
Sn = 33/2(300)
Sn = 33 * 150
Sn = 4950.