Vaibhav Beriwal Posted April 26, 2014 Share Posted April 26, 2014 1!-2!+3!-4!+... -98!+99!-100! Quote Link to comment Share on other sites More sharing options...
deon Posted April 26, 2014 Share Posted April 26, 2014 Notice that k! mod 10 = 0 for all k=5,6,..,100 since all these factorials include at least 2 and 5 as factors. So, you need only to compute 1-2+3!-4!=-19. Moreover, 1!-2! Quote Link to comment Share on other sites More sharing options...
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