Tackle the articulation, Using character: cos (θ + ϕ) = cosθ cos ϕ – sinθ sin ϕ

= 4/5 x 12/13 – 3/5 x 5/13

= (48-15)/65

= 33/65

This infers cos (θ + ϕ) = 33/65

or on the other hand θ + ϕ = cos-1 (33/65)

Returning the worth of θ and ϕ, we get

cos−1( 4/5)+ cos-1(12/13) + cos-1(33/65)

Henceforth Proved.