Cho 1 duong thang cat 2 duong thang song song,Hay chung minh rang:

a,Cac tia phan giac cua 1 cap goc so le (hoac so le ngoai thi song song voi nhau

b,Cac tia phan giac cua 1 cap goc dong vi thi song song voi nhau.

c,Cac tia phan giac cua 1 cap goc trong cung phia(hoac ngoai cung phia) thi vuong goc voi nhau.

hai duong thang a va b cat duong thang c va trong cac goc tao thanh co mot cap goc trong cung phia bu nhau. vi sao duong thang a song song voi duong thang b

Chung to rang neu hai duong thang song song thi cac tia phan giac cua moi cap goc dong vi song song voi nhau.

Tam giac ABC,diem I thuoc AB sao cho \(\frac{AI}{AB}=\frac{1}{3}\),qua I ke cac duong thang song song BC,AC lan luot cat AC,BC tai D,E. Chi ra cac cap tam giac dong dang,viet cac cap goc bang nhau,cac ti so dong dang tuong ung

Phat bieu nao sau day la dung:

A) Hai duong thang cung song song voi mot duong thang thi chung song song voi nhau

B) Chi co mot duong thang song song voi duong thang da cho

C) Chi co mot duong thang di qua mot diem song song voi duong thang da cho

D) Tat ca cac phat bieu tren deu sai

tren mot duong thang song song lay n diem a1,a2,a3,....an .qua cac diem nay ve cac duong thang song song voi nhau .tinh gia tri cua n de trong hinh co dung 100 tia

cho 2012 duong thang trong do khong co 2 duong thang nao song song va khong co 3 duong thang nao dong quy . Tim so giao diem cua cac duong thang ay

Cho goc xOy , tren Ox lay cac doan thang OA=AB=BC . Tu A,B,C ke cac duong thang song song voi nhau, cac duong thang nay cat nhau tai cac diem tuong ung A',B',C'. CMR :OA'=A'B'=B'C'

tu giac ABCD co AB=BC=CD .hai duong cheo AC va BD cat nhau tai N . cac duong thang AB va CD cat nhau tai M . duong thang di qua B song song voi CD va duong thang di qua C song song voi AB cat nhau tai P