`a) BD=` $\sqrt{AB²+BD²}$`= 25 (cm)`
`DB. BD= AB²` `text{⇒ OB= 9 (cm) ⇒ OD= 25- 9= 16 (cm)}`
`b)` diện tích hình thang: $\dfrac{BD.AC}{2}$
`AC=` $\sqrt{AD²+CD²}$
`text{⇒ BD² (AD²+ CD²)= (AB+ CD)². AD² (diện tích²)}`
`text{⇒ CD=}` `80/2` `text{(cm) ⇒ AC=}` `100/3` `text{(cm)}`
`c) S=` $\dfrac{BD.AC}{2}$= `1250/3` `(cm²)`
$@Joo$