a. Xét tgiac MAB va tgiac MDC co :

MD = MA ( gt )

BM = MC ( AM la dg trung tuyen)

^AMB = ^DMC ( 2 góc đối đỉnh)

=> tgiac MAB = tgiac MDC ( c.g.c) (dccm)

b. => AB = DC ( 2 canh tuong ung )

=> ^MBA = ^MCD ( 2 goc tuong ung )

- Ta co : 15^2 = 9^2 + 12^2

=> BC^2 = AB^2 + AC^2

=> tgiac ABC vuong tai A

Do BA vuog goc vs AC => DC vuog goc vs AC ( t/c quan he tu vuog goc den song song )

Ma ^MBA = ^MCD (CMT) => DC song song AB

Xet tgiac CKD va tgiac AKB co ;

AB = DC (CMT)

KC=KA (K la trung diem AC)

^BAK = ^DCK = 90o

=> tgiac CKD = tgiac AKB ( 2 cgv)

=> KD= KB ( 2 cah t.ung)