Answer:
Proof:
In ΔCDB
∵ The sum of all angles of triangle = 180°
∴ m∠B + m∠C + m∠D = 180°
R3. Their sum = 180 (They form straight angle)
S4. ∠B + ∠A = 180°
R5. Triangle angle sum theorem (Sum of all angles of triangle = 180°)
