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iceghost

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Qua A vẽ d // BC

Ta có : $\hat{A}_1 + \hat{A}_2 + \hat{A}_3 = 180^o$ ( kề bù )
Mà $\hat{A}_2=\hat{B}_1$ ( so le trong, d // BC )
$\hat{A}_3 = \hat{C}_1$ (so le trong, d // BC )
$\implies$ $\hat{A}_1+\hat{B}_1+\hat{C}_1=180^o$
Vậy tổng ba góc trong $\triangle$ bằng $180^o$
 
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