Find the greatest prime divisor of the value of the arithmetic series 1 + 2 + 3 + \dots + 135 + 136 + 137 + 138 + 139 + 140.
take the first and last number. sum them
\(140+1=141\)
multiply by half the number of total numbers. \(140/2=70\)
we thus have \(141*70=9870 \)
prime factorize 9870. \(9870 = 2 * 3 * 5 * 7 * 47\)
take largest prime number --> 47
47 is the answer.
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