1. The problem statement, all variables and given/known data If 1/2<=x<=1 , then cos-1[ x/2 + (√(3-3x2))/2] + cos-1 x is equal to? 2. Relevant equations 3. The attempt at a solution Substituting x=cosθ = cos-1(cosθ) + cos-1 [ cos(θ)/2 + (√(3(1-cos2(θ)))/2] = θ + cos-1 [cos (π/3) cos θ + sin (π/3) sinθ] = θ + cos-1 [cos (θ -(π/3)] = 2θ -(π/3) = 2 cos-1x -(π/3) But the correct answer according to the book is π/3. Please help me understand what I've done wrong.