Explicație pas cu pas:
6) 12^(n+1)+3^n×2^2n-12^n×3 = 12^n×12+3^n×4^n-12^n×3 = 12^n×12 +12^n-12^n×3 =12^n(12+1-3) =12^n×10
x=12^n×10/10 => x=12^n si cum n€N*
=> x€N*
7)(15-k)/3 €N => 3|(15-k) => k€{0,3,6,9,12,15}
0+3+6+9+12+15 =3+6+9+12+15 =3(1+2+3+4+5) =3× 5×6/2 =3×5×3=45
