Тригонометрия. 3arccos x + 3arccos x-1 x+10=0 (-1 - это степень)

3arccosx + 3arccos⁻¹ x + 10 = 0
3arccosx + 3/arccos x + 10 = 0  умножим на arccosx 
3arccos²x + 10arccosx + 3 = 0
arccosx = t
3t² + 10t + 3 = 0
D = 100 - 4*3*3 = 64
t₁ = (- 10 - 8)/6
t₁ = - 3
t₂ = (- 10 + 8)/6
t₂ = - 1/3
1)  arccosx = -3
x = cos(-3)
x₁ = cos(3)
2)  arccosx = - 1/3
x = cos(-1/3)
x₂ = cos(1/3)
ответ: x₁ = cos(3);  x₂ = cos(1/3)