a = 6^1996 + 5^1997 - 4^1998
u(4^1998) = u[(4^2)^999] =u(16 ^999) = 6
u( 6^1996 + 5^1997 - 4^1998) =u(6 + 5 - 6) = 5 ⇒ a ⋮ 5
___________________________________________________
b = 1•2•3• 4 •5• ... •1997 + 5
1•2•3• 4 •5• ... •1997 ∈ M₅ ⇒ 1•2•3• 4 •5• ... •1997 + 5 ∈ M₅ ⇒ b ⋮ 5