23:∠B=∠C

24: 3, right

25: ∠BFE=∠CEF Proof: ∠BDF=∠CDE, ∠BFE=∠CEF, BD=CD, ∴△BDF≌△CDE.

26:

27:∵∠cbe=∠cde∴∠cba=∠dba,∴△acb≌△adb(asa)

28:ad=eb,∴ab=de,bc∥df∴∠cbd=∠bdf,∴∠cba=∠fde,∴△cba≌△edf,∴ac=ef