There are 100 numbers in the range 100-199. Find the smallest and largest multiple of 13 in this range. We have
104 = 8•13
195 = 15•13
so there are 15 - 8 + 1 = 8 multiples of 13 between 100 and 199.
Then the sum we want is
[tex]\displaystyle \sum_{k=100}^{199} k - \sum_{\ell=8}^{15}13\ell[/tex]
or equivalently,
(100 + 101 + 102 + … + 199) - 13 (8 + 9 + … + 15)
To compute these sums, recall the following formula:
[tex]\displaystyle \sum_{i=1}^n i = 1 + 2 + 3 + \cdots + n = \frac{n(n+1)}2[/tex]
Then
[tex]\displaystyle \sum_{k=100}^{199} k = \sum_{k=1}^{199} k - \sum_{k=1}^{99} k = \frac{199\cdot200}2 - \frac{99\cdot100}2 = 14,950[/tex]
and
[tex]\displaystyle \sum_{\ell=8}^{15} 13\ell = 13 \left(\sum_{\ell=1}^{15} \ell - \sum_{\ell=1}^7 \ell\right) = 13 \left(\frac{15\cdot16}2 - \frac{7\cdot8}2\right) = 1,196[/tex]
which means the sum we want is 14,950 - 1,196 = 13,754.
