Find the sum of all numbers between 100 and 900 which are multiples of 7?
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the first no after 100 divisible by 7 is 105 and last before 1000 is 994.
thus,
a = 105
l = 994
d = 7 (common difference)
n= ?
From 994, n can be obtained.
an = a + (n-1)d
994 = 105 + (n-1)7
994-105/7 = n-1
127 = n-1
n=128
Now Sum of 128 terms= n/2 (a + l)
=128/2 (105 + 994)
=64 x 1099
=70336
Thus, the answer is 70336.
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