I just can't see it at the moment.

Find the sum of all positive integers less than 10,000 not divisible by seven.

Thanks guys.

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- Sep 11th 2007, 01:58 AMBartimaeussum of multiples
I just can't see it at the moment.

Find the sum of all positive integers less than 10,000 not divisible by seven.

Thanks guys. - Sep 11th 2007, 02:12 AMred_dog
From the sum of all positive integers less than 10000, substract the sum of all positive integers which are divisible by 7.

$\displaystyle \displaystyle\sum_{k=1}^{10000}k-\sum_{k=1}^{1428}7k$

and use $\displaystyle \displaystyle\sum_{k=1}^nk=\frac{n(n+1)}{2}$