People's education edition seventh grade mathematics textbook volume 2 90-9 1, all the answers.

8. according to the meaning of the question, in △DAC

∠DAC = 180-∠C-∠ADC = 180-70-90 = 20

∠BAC = 180-∠BAC-∠C = 180-50-70 = 60

AE is ∠BAC bisector.

∴∠EAC=∠BAE= 1/2∠BAC=25

BF is ∠ABC bisector.

∴∠abo=∠obc= 1/2∠abc= 1/2*60 = 30

At △ABO

∠BOA = 180-∠BAE-∠ABO = 180-25-30 = 125

9. According to the meaning ∠ b+∠ c =180-∠ a =180-100 = 80.

∵∠ 1=∠2,∠3=∠4

∴∠2+∠4= 1/2(∠B+∠C)=40

∴∠x= 180-(∠2+∠4)= 180-40 = 140

10, according to the meaning of the question, BE and CF are the bisectors of ∠ ABC and ∠ ACB respectively.

∴∠GBC=∠ABG= 1/2∠ABC

∠GCB=∠ACG= 1/2∠ACB

At △GBC

∠BGC = 180-∠GBC-∠GCB = 180- 1/2∠ABC- 1/2∠ACB = 180- 1/2(∠ABC+∠ACB)

∠∠A = 180-(∠ABC+∠ACB)

∴∠ABC+∠ACB= 180 -∠A

∠∠BGC = 180- 1/2(∠ABC+∠ACB)

∴∠bgc= 180- 1/2( 180-∠a)

= 180 -90 +∠A

=90 +∠A