Đáp án:
a) Ta có: $\widehat A + \widehat B + \widehat C = {180^0}$
Theo bài ra ta có:
$\begin{array}{l}
\widehat A.3 = \widehat B.5 = \widehat C.7\\
\Leftrightarrow \dfrac{{\widehat A}}{{35}} = \dfrac{{\widehat B}}{{21}} = \dfrac{{\widehat C}}{{15}} = \dfrac{{\widehat A + \widehat B + \widehat C}}{{35 + 21 + 15}} = \dfrac{{{{180}^0}}}{{71}}\\
\Leftrightarrow \widehat A = \dfrac{{{{6300}^0}}}{{71}},\widehat B = \dfrac{{{{3780}^0}}}{{71}},\widehat C = \dfrac{{{{2700}^0}}}{{71}}\\
Vậy\,\widehat A = \dfrac{{{{6300}^0}}}{{71}},\widehat B = \dfrac{{{{3780}^0}}}{{71}},\widehat C = \dfrac{{{{2700}^0}}}{{71}}\\
b)\\
\dfrac{{{{6300}^0}}}{{71}} > \dfrac{{{{3780}^0}}}{{71}} > \dfrac{{{{2700}^0}}}{{71}}\\
\Leftrightarrow \widehat A > \widehat B > \widehat C\\
\Leftrightarrow BC > AC > AB
\end{array}$
c)Cạnh BC dài nhất
Cạnh AbB ngắn nhất
