Answer:
With points A, B and C a right triangle is formed.
Given that BC ⊥ CA, then m∠C = 90°.
The addition of the angles of a triangle is equal to 180°, therefore:
m∠A + m∠B + m∠C = 180°
Substituting with m∠C = 90°:
m∠A + m∠B + 90° = 180°
m∠A + m∠B = 180° - 90°
m∠A + m∠B = 90°
