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Theo tính chất hai tiếp tuyến cắt nhau ta có:
DM = DB, EM = EC, AB = AC
Chu vi ΔADE:
CΔADE = AD + DE + AE = AD + DM + ME + AE = AD + DB + EC + AE = AB + AC = 2AB (đpcm)
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\(d_1//d_2\Leftrightarrow\hept{\begin{cases}a=a'\\b\ne b'\end{cases}}\)
\(\Leftrightarrow\hept{\begin{cases}m-1=2m\\5\ne-4\end{cases}}\Leftrightarrow\hept{\begin{cases}-1=2m-m\\5\ne-4\end{cases}}\Leftrightarrow\hept{\begin{cases}m=-1\\5\ne-4\end{cases}}\)