Question

find the sum of all natural numbers between 100 and 1000 which are multiples of 6
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Answer 1

Natural numbers b/w 100 and 1000 which are multiples of 5 are:

105, 110, 115, .*.., 990, 995.

a= 105, d 5

an = 995

a + (n 1)d = 995

105 + (n 1)5 = 995

(n-1)5 = 890

n -1 = 178

n = 179

Hence,

S. = (2a + (n 1)d]

179 2 S179 [210 + 890

179

S179 2 x 1100 = 98450