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Equation with factorial (Posted on 2018-10-20)

Solve in integers for x and y: 6(x! + 3) = y^2 + 5

No Solution Yet Submitted by Ady TZIDON

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soln

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x must be less than 5 (it is from choices: 0,1,2,3,4). Why?

x>=5 --> x! is a multiple of 10, and so is 6(x!). The equation:

6(x!)=y^2 -13 means that for x>=5, y^2 must end in 3, which is impossible for integers.

What works is (2,5) and (3,7) and also (2,-5) and (3,-7)

Edited on October 21, 2018, 3:32 am

Posted by Steven Lord on 2018-10-20 11:09:48

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