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+1
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avatar+616 

What is the units digit of the sum of   Thanks in advance.

MIRB16  Feb 14, 2018
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 #1
avatar+85673 
+1

 

 

1!  =  1

1! + 2!  =  1 + 2 = 3

1! + 2! + 3!  =   3 + 6  = 9

1! + 2! + 3! + 4! = 9 + 24  =  33

1 + 2! + 3!  + 4!  + 5!  = 33 + 120  =  153

 

Note that every successive factorial  added to his sum  will include  2 and 5  as multipliers...so every successive factorial will end in 0.....so

 

1! + 2!  + 3!  + 4! + 5!  .....+ 1000!   will   have a units digits sum  of 3

 

 

cool cool cool

CPhill  Feb 14, 2018

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