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# More arithmetic series.... watch

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1. Ok, having trouble with a different question if anyone can help.

Find the sum of the integers, between 1 and 300 inclusive, not divisable by 3.

Where do I start. Do i need an equation for a number not divisable by 3?

Thanks

Louise
2. You need an equation for the sum of the numbers divisable by 3. You can then find out the sum of all the integers between 1 and 300 and subtract the some of the numbers divisable by 3.
3. (Original post by Louise_1988)
Ok, having trouble with a different question if anyone can help.

Find the sum of the integers, between 1 and 300 inclusive, not divisable by 3.

Where do I start. Do i need an equation for a number not divisable by 3?

Thanks

Louise
Don't you calculate the sum of all number sbetween 1 and 300 inclusive and then subtract the sum of the numbers divisable by 3 from the overall sum
4. (Original post by zazy)
Don't you calculate the sum of all number sbetween 1 and 300 inclusive and then subtract the sum of the numbers divisable by 3 from the overall sum
Yer, I think I will do that, seems so ovbious now it has been pointed out!!
5. (Original post by Louise_1988)
Yer, I think I will do that, seems so ovbious now it has been pointed out!!
W00t? That's what I said! I feel so unloved.
6. (Original post by Zapsta)
W00t? That's what I said! I feel so unloved.
Sorry, it was intended for you as well. I am very grateful.

7. (Original post by Zapsta)
You need an equation for the sum of the numbers divisable by 3. You can then find out the sum of all the integers between 1 and 300 and subtract the some of the numbers divisable by 3.
Did i say the same thing???*blushes out of embarrasment8 sorry
8. (Original post by zazy)
Did i say the same thing???*blushes out of embarrasment8 sorry
Its always useful to have a second opinion!!!
9. a possibly simpler method uses the concept that the proof of the arithmetic series uses and that is to group the numbers like so...

(1 + 299) + (2 + 298) + (4 + 296) + ... + (149 + 151)

Each individual sum adds up to 300 so all we need to know is how many there are. Well between 1 and 300, 200 numbers are not divisible by 3 so there are 100 pairs of integers as above. So the sum is 300 x 100 = 30000

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