1. Solution: ∫∠2+∠3 = 180.

∴a∥b (the internal angles on the same side are complementary and the two straight lines are parallel)

∵∠3=∠4

∴c∥d (same angle, two parallel straight lines)

∫a∨b

∴∠ 3 =∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠

∵∠3+∠5= 180 ∠3=60

∴∠5= 120

3. Use parallel lines with equal internal angles: two mirrors are parallel, so 90-∠2=90-∠3, so ∠2=∠3, then ∠1+∠ 2 = ∠ 3+∠.