1) f(x) = 2x³ + 12x²
f'(x) = 2(x³)' + 12(x²)' = 6x² + 24x
f'(x) < 0
6x² + 24x < 0
x² + 4x < 0
x(x + 4) < 0
+ - +
________₀__________₀_________
- 4 0
x ∈ (- 4 ; 0)
2) f(x) = 9x² + 72x
f'(x) = 9(x²)' + 72(x)' = 18x + 72
f'(x) = 0
18x + 72 = 0
x + 4 = 0
x = - 4