20 12 The answer in the ninth grade math textbook is P39, the second question of Beijing Normal University Edition. The question of finding the point F on the bisector of the angle should be answered in detail. Why will this be proved? 2. Proof: as shown in Figure F-5.

FG⊥AD at point g, FM⊥BC at point m, FN⊥AE at point n,

∵∠ 1=∠2,

∴GF=MF

∫∠3 =∠4

∴MF=FN

∴GF=FN

Point F is on the bisector of ∠DAE.

3. Prove that (1)∵PC⊥OA is at point C and PD⊥OB is at point D,

∴∠OCP=ODP=90

∠∠AOP =∠BOP，OP = OP。

∴△OCP=△ODP(AAS)

∴OC=OD

(2), OC = OD, and OP is the bisector of ∠AOB.

Perpendicular bisector whose op is CD.

∴