Project Euler Problem 1

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問題

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 all the multiples of 3 or 5 below 1000.

ソース

sum = 0
(1...1000).each{|x|
	sum += x if x % 3 == 0 || x % 5 == 0
}
puts sum

解答

233168

感想

コレは激簡単！
これから頑張るぞ〜

Project Euler Problem 2

問題

Each new term in the Fibonacci sequence is generated by adding the previous two terms.
By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

Find the sum of all the even-valued terms in the sequence which do not exceed
four million.

ソース

sum = 0
x = 1
y = 2
while x <= 4000000 do
	x, y = y, x + y
	sum += x if x % 2 == 0
end
p sum

解答

4613732

感想

なぜ2が入らないのかが未だに謎。。。

Project Euler Problem 3

問題

The prime factors of 13195 are 5, 7, 13 and 29.

What is the largest prime factor of the number 600851475143 ?

ソース

require 'mathn'

x = 600851475143
pv = x.prime_division
p pv[pv.size - 1][0]

解答

6857

感想

ライブラリ使って余裕！