$cos A= \frac{b^{2}+c^{2}-a^{2}}{2bc}= \frac{53}{80}$.
Suy ra sin A $\approx$ 0,75
S = $\frac{b.c.sin A}{2}$ = 15
$r=\frac{S}{p}=\frac{15}{9,5}\approx 1,58$
$cos A= \frac{b^{2}+c^{2}-a^{2}}{2bc}= \frac{53}{80}$.
Suy ra sin A $\approx$ 0,75
S = $\frac{b.c.sin A}{2}$ = 15
$r=\frac{S}{p}=\frac{15}{9,5}\approx 1,58$