Project Euler is a series of challenging mathematical/computer programming problems. Please see the site and rules before posting.

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Combinatorial counting

This question is about Project Euler 113: Working from left-to-right if no digit is exceeded by the digit to its left it is called an increasing number; for example, 134468. Similarly ...
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4answers
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Find the sum of all the multiples of 3 or 5 below 1000

How to solve this problem, I can not figure it out: If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of ...
3
votes
4answers
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How to calculate reflected light angle?

On a two dimensional plane, line $X$ is at an angle of $x$ radians and an incoming light travels at an angle of $y$ radians. How can I calculate the angle of the outgoing light reflected off of the ...
3
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2answers
217 views

Faulty logic when summing large integers?

This is in relation to the Euler Problem $13$ from http://www.ProjectEuler.net. Work out the first ten digits of the sum of the following one-hundred $50$-digit numbers. ...