Quadratic expression integer iterations

[Janosh89]

Why is the quadratic expression 20*x^2-1 only divisible by decimal numbers ending in -1 , or ending in -9 ?
Sorry! My profile is a semi-retired carer.Location London(South) . Male 59. Have rudimentary maths -interests Number Theory , Prime Numbers.

 

[DrClaude]

Hi Janos,

You will have to post the question in the relevant forum, in your case https://www.physicsforums.com/forums/general-math.73/