Same question was posted earlier .... I repeat my answer here ...
Question (2a):
Given that sum of triangle = 180°
∠1 + ∠2 + ∠3 = 180°
∠3 = 180° - ∠1 - ∠2 ---------------- (1)
Given that sum of angles on the adjacent line = 180°
∠3 + ∠4 = 180°
∠4 = 180° - ∠3 ------------------------(2)
Sub (1) into (2)
∠4 = 180° - (180° - ∠1 - ∠2 )
∠4 = 180° - 180° + ∠1 +∠2
∠4 = ∠1 +∠2 (proven)
Question (2b):
∠4 = 52° + 25°
∠4 = 77°
