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.