a) $\text{Vì ΔABC vuong ở A nên :}$ $\\$ $\widehat{B}$ $+$ $\widehat{C}$ $=$$90^o$ $\\$ ⇔$51^o$ $+$ $\widehat{C}$ $=$$90^o$ $\\$ ⇔ $\widehat{C}$ $=$$39^o$ ( đpcm) $\\$ $\text{ Ta có : } $ $\\$ $AC$ = $AB$ . $tan B$ = $3,8 . tan 51^o$ = $4,693(cm) (đpcm) $$\\$ ΔABC vuông ở A : $\\$ $BC²=AB²+AC²( Pytago)$ $\\$ $⇔BC² = 3,8² + 4,693² $ $\\$ $⇔BC =6,039 (cm) (đpcm)$ $\\$ b) $\text{Vì ΔABC vuong ở A nên :}$ $\\$ $\widehat{C}$ $+$ $\widehat{B}$ $=$$90^o$ $\\$ ⇔$60^o$ $+$ $\widehat{B}$ $=$$90^o$ $\\$ ⇔ $\widehat{B}$ $=$$30^o$ $( đpcm)$ $\\$ $\text{ΔABC vuông tại A có $\widehat{B}= 30^o$:}$ $\\$ $⇒ AC = \frac{BC}{2} =\frac{11}{2}= 5,5 (cm) $$\\$ $ \text{Vì ΔABC vuong ở A nên :}$ $\\$ $BC²= AB²+AC² (Pytago)$ $\\$ $⇔11²= AB² + (5,5)²$ $\\$ $⇔AB²=11²- (5,5)^2$ $\\$ $⇔AB= 9,526(cm)(đpcm)$
