单项选择题
A.1/5 B.2/5 C.3/5 D.4/5
A.6 B.12 C.18 D.36
A.76 B.75.8 C.75.5 D.76.5
离散型随机变量X的分布律为: 则a等于()
A.1/4 B.1/3 C.1/2 D.1
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