Reason: In △BDC, ∫∠BCD = 90, ∠ 1=∠2,

∴∠ 1 =∠ 2 = 90 ÷ 2 = 45 degrees

∫≈ 1 =∠3, ∴∠3=45 degrees,

∴∠doc= 180-(∠ 1+∠3)= 180-2×45 = 90，

∴CO⊥DB，

∴CO is the height of △BCD.

(2) 5 = 90-4 = 90-60 = 30.

(3)∠CDA =∠ 1+∠4 = 45+60 = 105，

∠DCB=90，

∠DAB=∠5+∠6=30 +30 =60，∠ ABC= 105

9. First calculate the degrees of the internal angles of the regular pentagon, and then calculate ∠ 1 = ∠ 2 = ∠ 3 = ∠ 4 = 36, so as to calculate X = 108-72 = 36. Solution: Because the Pentagon.

∠∠DAB =∠DBA，

∴∠ 1=∠2=∠3=∠4=( 180 - 108 )÷2=36 ,

∴x= 108 -72 =36。