Sana: 201 y. Mavzu: ko'pburchaklar darsning maqsadi

2-teorema:
(2-alomat.) Agar to'rtburchakning diagonallari keshishsa va kesishish nuqtasida teng ikkiga bo'linsa, bu to'rtburchak parallelo­grammdir.
I sbot. ABCD to'rtburchakda O nuqta AC va BD diagonallarining kesishish nuqtasi hamda AO= OC va BO = OC tengliklar bajariladi (28- rasm). Uchburchaklar tengligining 1-aloma-tiga ko'ra AOB va COD uchburchaklar teng (AO=OC, B0= OD - shartga ko'ra, AOB= COD - vertikal burchaklar), shuning uchun AB= CD va 1= 2. 1 va 2 burchaklarning teng- ligidan, AB|| CD (to'g'ri chiziqlarning parallellik alomatiga ko'ra) kelib chiqadi.
Shunday qilib, ABCD to'rtburchakda AB va CD tomonlar teng hamda parallel, demak, parallelogrammning 1-alomatiga ko'ra ABCD to'rtburchak-parallelogrammdir.
Parallelogrammning yana quyidagi alomatlari bor:

3. 
   Agar to'rtburchakning qarama-qarshi tomonlari jufti-jufti bilan teng bo'lsa, bu to'rtburchak parallelogrammdir.
   
4. 
   Agar to'rtburchakning qarama-qarshi burchaklari jufti-jufti bilan teng bo'lsa, bu to'rtburchak parallelogrammdir.
   
5. 
   Agar to'rtburchakning ixtiyoriy bir tomoniga yopishgan burchaklari yig'indisi 180° ga teng bo'lsa, bu to'rtburchak parallelogrammdir.
   

4. Yangi mavzuni yoritish va Yangi mavzuni mustahkamlash:
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