cho duong tron (o) duong kinh AB va tiep tuyen Bx cua (o) tren Bx lay diem M, MA cat (o) tai S goi N la diem chinh giua cung nho AS. nối ON cắt AS tai I chung minh
a) goc ABM = goc ASB
b) BN la tia phan giac cua góc ABS
c) ON song song BS
d) tu giac MIOB noi tiep