Một hộp chứa 9 chiếc thẻ được đánh số từ 1 đến 9. Lấy ngẫu nhiên 3 chiếc thẻ từ hộp. Tính xác suất để tổng các số ghi trên 3 chiếc thẻ được lấy ra là một số lẻ.
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Số phần tử của không gian mẫu: \(n(\Omega)=C_9^3=84\).
Gọi A là biến cố "tổng các số ghi trên 3 chiếc thẻ được lấy ra là một số lẻ".
Ta có \(n(A)=C_{5}^{3}+C_{4}^{2}.C_{5}^{1}=40\).
Xác suất để tổng các số ghi trên 3 chiếc thẻ được lấy ra là một số lẻ là:
\(p(A)=\frac{n(A)}{n(\Omega)}=\frac{40}{84}=\frac{10}{21}\).
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