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Mathematical problems in the first volume of the eighth grade. Solve! ! ! !
Proof: (1) ∵ ab = ad; AE = AC∠BAE=∠DAC= 120? .

∴⊿bae≌⊿dac(sas),∠abm=∠adn;

And AB = AD∠BAM=∠DAN=60? .

∴⊿BAM≌⊿DAN(ASA),AM=AN.

(2)∵AM=AN (authentication); ∠MAN= 180? -∠BAD-∠CAE=60? .

∴⊿MAN is an equilateral triangle, ∠AMN=∠MAB=60? .

∴MN∥BC.