Since ∠ COA is complementary to ∠ AOF , m ∠ COA m ∠ AOF = 90 ∘ . Since ∠ EOB forms a vertical angle with ∠ AOF , they are congruent by the vertical angle theorem. By the substitution property of equality, m ∠ COA 40 ∘ = 90 ∘ . Applying the subtraction property of equality gives m ∠ COA = 50 ∘ . What is missing from the proof?