[Toán 8] c/m ABCD là hình thang

V

vy000

Ta có hệ:[
$\begin{cases}\widehat A+ \widehat B+ \widehat C+ \widehat D = 360^o\\ \widehat A+ \widehat D = \widehat B+ \widehat C \end{cases}$

$\Rightarrow \widehat A+ \widehat D=\widehat C+ \widehat B=180^o$

$\Rightarrow AB//CD$
 
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