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What is the greatest prime factor of 1+2+3+。。。+40?

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by Max@Math Revolution » Fri Jun 17, 2016 11:28 pm
What is the greatest prime factor of 1+2+3+。。。+40?
A. 17
B. 29
C. 31
D. 37
E. 41

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by Max@Math Revolution » Mon Jun 20, 2016 4:33 pm
We can calculate 1+2+3+.....+n=n(n+1)/2. Then, 1+2+3+。。。+40=40(40+1)/2=20(41)=(2^2)(5)(41). Hence, the biggest prime factor is 41 and the correct answer is E.
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