mk giai dc rui do. ban tu ve hinh nha.
kéo dài bk OA, căt dt tại K.
TA CO: ACB- ABC= BKA- ABC (1)( VI ACB= BKA)
MA BKA= ABK- BAK= 90- BAK, THAY VAO (1) SUY RA
ACB-ABC= 90-BAK- ABC
= 90- (BAK+ ABC)
MA BAK+ ABC= (sd cungAC+ sd cungBK)/2= BOK(t/chat góc trong).
suy ra ACB- ABC= 90- BOK= 90- AOH= OAH SUY RA DPCM