2011SP-I-16
答案:(a) $\dfrac{10}{21}$ (b) $\dfrac{11}{21}$
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= & 1- \left( \dfrac{2
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= & \dfrac{P^6_6 \tim
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= & \dfrac{2}{10} \\
=
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= & \dfrac{6}{20} \\
=
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= & C^{20}_5 – C
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= & \dfrac{3}{6}\tim
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=
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= & \dfrac{9!}{10!} \
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= & \dfrac{C^4_2}{C^
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= & 3! \times 5! \\
= &
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= & C^{13}_2 \times C