Junior high school mathematics questions for positive integer n formula (3N + 1) (3n-1) - (3-N) (3-N) value is not a multiple of 10, try to explain the reason

Junior high school mathematics questions for positive integer n formula (3N + 1) (3n-1) - (3-N) (3-N) value is not a multiple of 10, try to explain the reason

Then (3 + n) (3-N)
Original formula = 9N & sup2; - 1-9 + n & sup2;
=10N²-10
=10(N²-1)
So it's a multiple of 10

For any positive integer n, try to explain that the value of integer (3N + 1) (3n-1) - (3-N) (3 + n) must be a multiple of 10
Please explain that when n = 1,

（3n+1)(3n-1)-(3-n)(3+n)
=9n^2-1-(9-n^2)
=9n^2-1-9+n^2
=10n^2-10
=10 (n ^ 2-1) is a multiple of 10
When n = 1, (3N + 1) (3n-1) - (3-N) (3 + n) = 0 is a multiple of 10

Quadratic - (M + n) (m-2n) - 3N (n-m) of 2 (m-n)

2(m-n)²-(m+n)(m-2n)-3n(n-m)
=2(m-n)²+3n(m-n)-(m+n)(m-2n)
=[2(m-n)+3n](m-n) -(m+n)(m-2n)
=(2m+n)(m-n) - (m+n)(m-2n)
=2m²-mn-n²-（m²-mn-2n²）
=2m²-mn-n² - m²+mn+2n²
=m²+n²