Exercise 20 Page 305

Using the Supplement Theorem, we get the following two relations. m∠ 1 + m∠ 2 &= 180 m∠ 3 + m∠ 4 &= 180 By adding these two equations we obtain what we wanted to prove. m∠ 1 + m∠ 2 + m∠ 3 + m∠ 4 = 360 Notice that ∠ 1 and ∠ 4, as well as ∠ 2 and ∠ 3, also form a linear pair.