0 ! + 1 ! + 2 ! + 3 ! + … + 2 0 1 3 ! + 2 0 1 4 !
What are the last two nonzero digits of the number above?
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Splendid. By knowing its trailing zeros, one could easily "terminate" the irrelevant terms to find the last few digits of 1 ! + 2 ! + … + 2 0 1 4 ! .
Nicely done sir!Did it the same way as yours.
The Factorial of all the numbers starting from 10 has the last two digits (atleast) as 0 and 0. This means we have to find the last two digits of
n = 0 ∑ 9
which is
n = 0 ∑ 9 ( m o d 1 0 0 ) ≡ 1 + 1 + 2 + 6 + 2 4 + 2 0 + 2 0 + 4 0 + 2 0 + 8 0 ( m o d 1 0 0 ) ≡ 2 1 4 ( m o d 1 0 0 ) = 1 4
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For n ≥ 1 0 we have that n ! has the factors 2 , 5 and 1 0 , and thus its last two digits will both be 0 . Thus we just need to determine
( n = 0 ∑ 9 n ! ) ( m o d 1 0 0 ) = ∑ n = 0 9 ( n ! ( m o d 1 0 0 ) )
≡ ( 1 + 1 + 2 + 6 + 2 4 + 2 0 + 2 0 + 4 0 + 2 0 + 8 0 ) ( m o d 1 0 0 )
≡ 2 1 4 ( m o d 1 0 0 ) = 1 4 .