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+∠.