✔ 最佳答案
3.
BC=BD
∴∠BCD=∠BDC (等腰)
AB=AC
∴∠ABC=∠ACB (等腰)
36° + ∠ABC +∠ACB = 180°
36° + ∠ABC +∠ABC = 180°
∴∠ABC = 72°
∠CBD
= 180° - ∠BCD - ∠BDC
= 180° - 72° - 72°
=36°
∠ABD = ∠ABC - ∠CBD = 72° - 36° = 36° ∎
4.
AD=AD
∠ADB=∠ADC
DB=DC
∴ΔADB=ΔADC (SAS)
∴∠BAD=∠CAD ∎
5.
∠ADC=∠ABC
AD=AB
AC=AC
∴ΔADC=̃ΔABC (RHS)
∠DCA=∠BCA
DC=BC
EC=EC
∴ΔBEC=Δ̃DEC
∴DE = BE
∠BEC=∠DEC
180° = ∠BEC+∠DEC = 2∠BEC
∴∠BEC = 90° ∎