3n^2 - n + 4 = 3n^2 + 12n - 13n + 4 = 3n( n + 4 ) - 13n - 52 + 56 = 3n( n + 4 ) - 13( n + 4 ) + 56 = ( n + 4 )( 3n - 13 ) + 56;
Fractia ta devine : 3n - 13 + 56/ ( n + 4); ca ea sa fie nr. intreg => ( n + 4 ) ∈ D 56 => ( n + 4 ) ∈ { 1, 56, 2, 28, 4, 14, 7, 8 }; pt. ca n e nr. natural => n ∈ { 52, 24, 0, 10, 3, 4 }.
Bafta!