Menh de 1.1.8 cac tien de sau doc lap voi nhau:

I1: voi 2 diem A,B bat ky co duy nhat mot dt l chua ca A,B

I2: moi duong thang chua it nhat 2 diem

I3: ton tai 3 diem khong thang hang

P: voi moi diem A va voi moi duong thang l. ton tai nhieu nhat mot duong thang chua A va song2 voi l

CM:

* lay mo hinh thoa man I123 khong thoa man P

Lay tap hop gom 5 diem A,B,C,D,E va duong thang la nhung tap con gom 2 diem.

+ mo hinh nay t/m I123

+khong thoa man P do

{A,B}//{D,E}

{A,C}//{D,E}

* mo hinh thoa man I12, P khong thoa man I3

Lay tap gom 2 diem va duong thang la tap chua ca 2 diem do

+thoa man I1: qua 2 diem co duy nhat 1 dt

+thoa man I2:dt thang chua it nhat 2 diem

+thoa man P: voi 1 diem A nam tren dt khong con dt nao qua A song song voi dt thang da cho(ngoai dt da cho)

+khong thoa man I3: khong co 3 diem khong thag hang

*mo hinh thoa man I13, P khong thoa man I2

Lay tap gom 3 diem A,B,C cac duong thang gom cac tap con {A,B}, {A,C}, {B,C}, {A}

+duong thang {A} chua 1 diem=> khong thoa man I2

+ton tai 3 diem khong tang hang

+dt chua diem A song song voi {B,C}

* lay mo hinh thoa man I23, P khong thoa man I1

Lay tap gom 3 diem va khong co duong thang nao ca