Решите неравенство f' (x) < 0:

1) f (x) = 2x^4 - x

2) f (x) = 3 x^3 - 27x

3) f (x0 = 1/x - 2x - 1

4) f (x) = 1/x^2 + 54x + 3

помогите

1) f (x) = 2x^4 - x

f ' (x) = (2x^4 - x) ' = 2*4x^3 - 1 = 8x^3 - 1

8x^3 - 1 ≥ 0

8x^3 ≥ 1

x^3 ≥ 1/8

x ≥ 1/2 = 0,5

x ∈ [ 0,5; + беск)

2) f (x) = x^3-27x

f ' (x) = 3x^2 - 27

3x^2 - 27 ≥ 0 / : 3

x^2 - 9 ≥ 0

(x - 3) (x + 3) ≥ 0

x ∈ (- беск; - 3] ∨ [ 3; + беск)