x+ 1/x =3; ridicam la puterea a 3-a
(x+ 1/x)³=3³
x³+3•x²/x+3•x/x²+1/x³=27
x³+3(x+1/x)+1/x³=27
x³+3•3+1/x³=27
x³+1/x³=27-9
x³+1/x³=18
