∴∠∠∠1=∠ ANC Two straight lines are parallel, and the included angles are equal.
∵AB//CD is known.
∴∠
∴∠C=∠ 1 equivalent substitution
∫∣≈ 1 = 60 is known.
C = 60 equivalent substitution
2. Proof: ∠BAD=∠CAD
Ef⊥bc ∵ad⊥bc (known)
∴∠ EFD = _ _ ∠ ADC _ _ _ _ = 90 (vertical definition)
∴EF//AD (same angle, two parallel straight lines)
∴∠ E = _ _ ∠ CAD _ _ _ _ two straight lines are parallel and have the same angle)
∫≈ 1 =∠E (known)
∵EF//AD is known.
∴∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠∠873
∴∠BAD=∠CAD (equivalent substitution)
Let's ask two questions first.