Bonjour,
1) x³ < 27
x³ < 3³
x < 3
2) x³ < -8
x³ < (-2)³
x < -2
3) 1 + x³ > 65
x³ > 65 - 1
x³ > 64
x³ > 4³
x > 4
4) (2 + x)³ < -125
(2 + x)³ < (-5)³
2 + x < -5
x < -5 - 2
x < -7