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Nếu ko có 3 điểm nào thẳng hàng thì vẽ đc 45 đường thẳng và 90 tia
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a) Có tất cả n(n-1):2 đoạn thẳng
b)Ta có 55.2 = 110
Mà 110=11.(11-1)
-> n=11 Vậy n=11
Học tốt nhé bạn kich, cho mình đi