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 From factorial to power (Posted on 2020-08-31)
There are two distinct integer solutions to

x!+8=2y

a. Find both.
b. Prove no other solutions exist.

 No Solution Yet Submitted by Ady TZIDON No Rating

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 Analytical Puzzle Solution Comment 2 of 2 |
y cannot be 0, since this forces x! to be negative. This is a contradiction.
x=0 gives 2^y =9, which does not have an integer solution in y.
Therefore each of x and y is a positive integer.

Also,  we note that:
6!=720 = 16*45 = M(16)
Since all x! for x>=6 is a multiple of 6, it follows that for x>=6 and y>=4 each of x! and 2^y is a multiple of 16.
Accordingly,  x!=16P and 2^y= 16Q, whenever each of P and Q is a positive integer.
Substituting these values in the given equation, we have:
16P+8=16Q
=> 16(P-Q)=8
=> P-Q= 0.5,
So, lhs of the above equation is an integer but the rhs is not an integer.
Accordingly,  x<6
Then, substituting x=1,2,3,4,5 in turn, we note that:
x=4 yields, 2^y =32, giving: y=5
x= 5 yields, 2^y=128, giving: y=7
None of the other three values of x gives an integer value of y.

Consequently,
(x, y) = (4,5) and (5, 7) corresponds to all possible integer solutions to the equation under reference.

Edited on June 22, 2022, 12:48 am
 Posted by K Sengupta on 2022-06-22 00:08:14

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