Giải thích các bước giải:
a.Ta có: $AN\perp BC$
$\to \tan B=\dfrac{AN}{BN}\to BN=\dfrac{AN}{\tan B}$
Tương tự $CN=\dfrac{AN}{\tan C}$
$\to BC=BN+CN$
$\to BC=\dfrac{AN}{\tan B}+\dfrac{AN}{\tan C}$
$\to BC=AN\cdot (\dfrac1{\tan B}+\dfrac1{\tan C})$
$\to 11=AN\cdot (\dfrac1{\tan 38^o}+\dfrac1{\tan30^o})$
$\to AN\approx 3.65$
b.Ta có: $\sin C=\dfrac{AN}{AC}$
$\to AC=\dfrac{AN}{\sin C}\approx 7.3$