1
is divisible by
1
.
1
×
2
×
3
is divisible by
1
+
2
+
3
.
1
×
2
×
3
×
4
×
5
is divisible by
1
+
2
+
3
+
4
+
5
.
1
×
2
×
3
×
4
×
5
×
6
×
7
is divisible by
1
+
2
+
3
+
4
+
5
+
6
+
7
.
In general, is it true that 1 × 2 × ⋯ × n is divisible by 1 + 2 + ⋯ + n for all odd n ?
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You have a typo on the 2nd line
We know that S ( n ) = k = 1 ∑ n k = 2 n ( n + 1 ) .
If we now divide n ! by S ( n ) as such, S ( n ) n ! = n ( n + 1 ) 2 n ! = n + 1 2 ( n − 1 ) ! . Note that n is an odd integer and therefore n + 1 is even. Because of this, we can express n + 1 as 2 x where x ∈ Z + and therefore we can simplify our fraction to x ( n − 1 ) ! and since x < ( n − 1 ) it is contained within ( n − 1 ) ! and thus will divide into it. Therefore the answer is Yes .
Much easier solution that others.
The product will be equal to n! , then sum 1+2+..+n = n(n+1)/2. As n is odd (n+1) will be even, and when divided by 2 it, number comes less than 'n' and let it be 'x'.
Now n!/(n \times x), it will be always divisible.
Easy solution, especially for me
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Let us denote the odd integer n as O . P and S as product and sum of n respectively and we have ( P / S ) n ∈ O = 1 + 2 + 3 + ⋯ + n 1 × 2 × 3 × ⋯ n = 2 n ( n + 1 ) n ! ( P / S ) n ∈ O = n ( n + 1 ) 2 n ! = n + 1 2 ( n − 1 ) ! We can cleary note that ( n − 1 ) and ( n + 1 ) are even integers since n is an odd integer that implies that ( n − 1 ) ! = ( 2 m 1 ) ! and n + 1 = 2 m 2 where ( m 1 , m 2 ) ∈ N also m 2 − m 1 = 2 ( P / S ) n ∈ O = n + 1 2 ( n − 1 ) ! = 2 m 2 2 ( 2 m 1 ) ! ( P / S ) n ∈ O = m 2 ( 2 m 1 ) ! = m 2 ( 2 ( m 2 − 2 ) ) ! which is always divisible. Hence the answer is Yes .