What is the remainder when
1 ! + 2 ! + 3 ! + ⋯ + 5 0 !
is divided by 5 ! ?
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Cheers!! :)
Brilliant, this one really stumped me
C h e e r s ! !
5!,6!,7! Is divisible by 5! Thus dividing 5!+6!+7!...+50! By 5! Will leave a zero remainder. 1!+2!+3!+4! Will be the residue. Therefore,1+2+6+24=33(remainder)
All the terms after 4! have a factor 5! So the remainder of 1!+2!+3!+4!+...+50! is equal to 1!+2!+3!+4! = 33 . In other words, we can simplify this terms into the form 1!+2!+3!+4!+5!(n),,so the remainder will be determined by the terms before 5!(n) .It obvious the remainder is 33
We know 5 ! is the multiple of ( 5 + k ) ! where, k ∈ N So, it can be easily said that 6 ! , 7 ! , 8 ! … ∞ will be divided by 5 ! ... So, ( 1 ! + 2 ! + 3 ! + … + 5 0 ! ) ≡ ( 1 ! + 2 ! + 3 ! + 4 ! ) ( m o d 5 ! ) ...... So, the remainder is 1 ! + 2 ! + 3 ! + 4 ! = 3 3
We observe that in the series, after 4! the remainder is 0 in each case when divided with 5!.
Now we are left with 1! + 2! + 3! + 4!.
as 1! + 2! + 3! + 4! = 1 + 2 + 6 + 24 = 33,
the remainder will be 33.
also , 33 is congruent to 33 (mod 5!).
Notice that the terms from 5 ! to 5 0 ! all contain a 5 ! in their product, which means that their units digit are either 0 or 5. Theoretically speaking, if you add all the units digits of 5 ! to 5 0 ! , the sum will be divisive by 5 ! . This means that we only need to calculate the sum from 1 ! to 4 ! to see if it's divisible by 5. Adding the first four terms, we get 3 3 .
Your solution finds the remainder when divided by 10, and not when divided by 5!. How can we approach this problem if it wasn't multiple-choice?
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All the terms after 4! i.e. 5!, 6!, 7!... can be written in terms of 5!(k), where k is some natural number...So the remainder will be formed by the terms in the beginning before 5!, i.e. 1!+2!+3!+4!=33
CHEERS!!:)