In triangle ABC AC2=AB2+BC2−2(AB)(AC)cosθ AC2=62+42−2×6×4×cosθ AC2=52−48cosθ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(I) Similarly in triangle ADC, AC2=AD2+DC2−2×AD×DCcos(180∘−θ) AC2=32+52+2×3×5×cosθ AC2=34+30cosθ⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅(II) From equation (I) and (II), 52−48cosθ=34+30cosθ cosθ=