a^2(c^2-9)+4(c^2-9)= (c^2-9)(a^2+4)
X²-2x-8≤0
x1+x2=2 U x1*x2=-8⇒x1=-2 U x2=4
x∈[-2;4]
x²-4≥0
(x-2)(x+2)≥0
x=2 x=-2
x∈(-∞;-2] U [2;∞)
sin³x·cos x - cos³x·sin x = √3/8
sin x·cos x·(sin²x - cos²x) = √3/8
1/2 · sin 2x · (-cos 2x) = √3/8
2 · sin 2x · (-cos 2x) = √3/2
sin 4x = -√3/2
4x = (-1)ⁿ⁺¹·π/3 + πn, n ∈ Z
x = (-1)ⁿ⁺¹·π/12 + πn/4, n ∈ Z