\(AA'=c.sinB=b.sinC\Leftrightarrow\frac{c}{sinC}=\frac{b}{sinB}.\)
va\(BB'=c.sinA=a.sinC\Leftrightarrow\frac{c}{sinC}=\frac{a}{sinA}\)
\(\Leftrightarrow\frac{a}{sinA}=\frac{b}{sinB}=\frac{c}{sinC}\)
\(S_{\Delta ABC}=\frac{1}{2}.a.AA'=\frac{1}{2}.a.bsinC\)