用Python解答 ProjectEuler问题(1)

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有个很有意思的网站 ProjectEuler.net ，提出了200多道数学问题，要求读者用计算机求解，不限制所用的计算机语言。
（2008年11月）试着用Python做了几道，挺有意思的。

• Add all the natural numbers below one thousand that are multiples of 3 or 5.
  
• Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed four million.
  
• Find the largest prime factor of a composite number.
  
• Find the largest palindrome made from the product of two 3-digit numbers.
  
• What is the smallest number divisible by each of the numbers 1 to 20?
  

E001
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.
求1000以下，所有是3或5倍数的数之和。

def problem1():
a, b, c = 3, 5, 999
f = lambda x,lmt=c: x*(lmt/x)*(lmt/x+1)/2
return f(a)+f(b)-f(a*b)
if __name__=='__main__':
print str(problem1())

关键是 1000以内能被3整除的数之和是 3(1+2+3+...+333)