có số cái bắt tay kà:

7 x (7 -1) : 2 = 21

Đáp số: 21 cái bắt tay

la 92 day nhe

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Cho tam giác ABC nhọn có các đường cao AD,BE,CF cắt nhau tại H

CMR:\(\frac{AD}{HD}\)+\(\frac{BE}{HE}\)+\(\frac{CF}{HF}\)≥9 (không dùng bất đẳng thức)