1) Một phòng họp có 10 người, 10 người này bắt tay lẫn nhau. Hỏi có tất cả bao nhiêu cái bắt tay?

Câu hỏi này được treo giải thưởng trị giá +15000 vỏ sò bởi vanduc040902@gmail.com, đã hết hạn vào lúc 27-11-13 09:05 PM

áp dụng thẳng vào công thức ta có $C^{2}_{10}$  cái bắt tay
nay nhe!nguoi dau ten se co the bat tay voi 9 nguoi con lai nen co 9 cak.nguoi thu 2 ,thu3,thu 4,thu 5,thu6,........thu 10 lan luot co 8,7,6,5,4,,3,2,1.=>co 9+8+7+6+5+4+3+2+1=45.
cak khac ngan hon,vi co 10 nguoi nen se co 10 cai tay phai nen se co 10C2=45 cai bat tay

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