∠∠COB =∠AOB-∠AOC

∠∠AOB = 180，∠ AOC = 40。

∴∠COB= 180 -40 = 140

Oe division ∠BOC

∴∠COE= 1/2∠BOC=70

∠∠DOE =∠DOC-∠COE

∠∠Doc = 90，∠ COE = 70。

∴∠DOE=20

(2) Write the answer directly: 1/2α (that is, α, not A)

(3)① Conclusion: 2∠DOE=∠AOC

∠∠COB =∠AOB-∠AOC

Once again ∵OE division ∠COB

∴∠EOC= 1/2∠COB

= 1/2(∠AOB-∠AOC)

∵? ∠DOE=∠COD-∠EOC

=90 - 1/2∠COB

= 90- 1/2∠AOB+ 1/2∠AOC

= 90- 1/2 * 180+ 1/2∠AOC

∴2∠DOE=∠AOC

②2∠AOF+∠BOE = 1/2(∠AOC-∠AOF)

4∠AOF+2∠BOE=∠AOC-∠AOF

5∠AOF+2∠BOE = 2∠ doe

5∠AOF+∠COB=2∠DOE

5∠AOF+∠AOB-∠AOC=2∠DOE

5∠AOF+∠AOB=2∠DOE+∠AOC

5∠AOF+∠AOB=4∠DOE

∴5∠AOF+ 180 =4∠DOE

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