cos -1 (2x2 – 1) = 2 cos -1 x
Take x = cos θ
Where θ = cos -1 x
Here LHS = cos -1 (2x2 – 1)
By substituting the value of x
= cos -1 (2cos2 θ – 1)
It can be written as
= cos -1 (cos 2 θ)
= 2 θ
Substituting the value of θ
= 2 cos -1 x
= RHS
Hence, it is proved.