cos -1 (4x3 – 3x) = 3 cos -1 x, 1/2 ≤ x ≤ 1
Take x = cos θ
Where θ = cos -1 x
Here LHS = cos -1 (4x3 – 3x)
By substituting the value of x
= cos -1 (4 cos3θ – 3 cos θ)
So we get
= cos -1 (cos3θ)
= 3θ
By substituting the value of θ
= 3 cos -1 x
= RHS
Hence, it is proved.