Math Is Fun Forum

Ash900

Guest

Further maths: Summing the series help.

Its pretty simple i think but i have a habit of making a lot of mistakes and i cant seem to get this one right or spot what i'm doing wrong.

The answer is 1,900,000 and i managed to get 19,000,000...grr lol.

Find the sum of the cubes of the integers less that 100 which are not divisible by 5.

Any help would be appreciated. .

Thankyou

luca-deltodesco

Member

Registered: 2006-05-05

Posts: 1,470

Re: Further maths: Summing the series help.

easiset way, what is the sum of all the numbers, what is the sum of all the numbers that ARE divisable by 5, subtract:

sum of all the numbers:

sum of all multiples of 5:
99/5 = 19.8 -> 19 (there are 19multiples of 5)

24,502,500-4,512,500 = 19,990,000

you get the same answer irrespective of if you include 100 in the summation. but nevermind that.

using a brute force summation in a program, you get my answer so it seems the book/mark scheme/teacher is incorrect

i also have this exam coming up

Last edited by luca-deltodesco (2007-12-29 03:59:01)

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Ash900

Guest

Re: Further maths: Summing the series help.

Thank you very much .

Good luck with your exam .