cau 1
Xet tam giac abc vuong tai A, co Ah la duong cao ( gt)
Ta co: AB^2=BH*BC ( he thuc luong )
          AB^2=3.2*5
         AB^2=16
=> Ab= can 16 = 4(cm)
cau 2: 
Xet tam giac def vuong tai A
Ta co: EF^2 = DE^2+DF^2 ( pytago )
           EF^2 = 6^2 + 8^2
           EF^2 = 100
=> EF = 10
Ap dung he thuc luong vao tam giac def vuong tai a, duong cao ah (gt)
Ta co: DE*DF = DL*EF
           6*8       = DL*10
 =>DL 4.8 ( cm )
Cau 3:
Xet tam giac ABC vuong tai A
Ta co: BC^2=AB^2+AC^2 ( pytago)
          BC^2 = 12^2 + 9^2
          BC^2 = 225
=> BC= 15
Xet tam giac ABC vuong tai A, duong cao Ah ( gt)
Ta co: AC^2=CH*BC ( he thuc luong)
          9^2    = CH * 15
=>      CH    = 5.4 ( cm)