... A A C A H CA H= = = .0 ,25 • 2 01 3. .sin 120 2 2ABC a S AC BC∆= =0 ,25 • 2 2 2 0 2 2 . .cos 120 7 7AB AC BC AC BC a AB a= + − = ⇒ =• 2. 21 7ABCS a CHAB∆= =Suy ra: 0 2 21'sin30 ... a A C = =.0 ,25 Xét tam giác vuông AA’C ta được: 2 235' '7 a AA A C AC= − =. Suy ra: 3105. '14ABC a V S AA∆= =.0 ,25 Do ( )'/ / ' '/ / ' 'CC ... AA CC ABB A ,Suy ra: ( ) ( )( )( )( ) 21 ' , ' ', ' ' , ' '7 a d A B CC d CC ABB A d C ABB A CH= = = =.0,5V(1,5điểm)Ta có: ) (21 )()(3) (21 )(4 22 22 yxyxyxyxxyyx++≤−++⇔++≤++ 2 )(3) (21 ...