find the sum of all the number between 250 and 1000 which are the exactly divisible by 3
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The A.P. is 252, 255....999
hence a1 = 252, an = 999 and d = 3
an = a + (n-1) d
999 = 252 + 3n - 3
(solving it) n = 250
hence, sn = n/2 [2a + (n-1) d]
sn = 250/2 [ 504 + 747]
sn = 125*1251
therefor sn = 156375
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