简答题:(本题满分l0分)
已知离散型随机变量X的概率分布为

(1)求常数a；
(2)求X的数学期望EX及方差DX．

A 0 B 1 C 2 D 3

A -l B 0 C 1 D 2

A cosx B -cosX C 2+cosX D 2-cosx

A <img src="https://www.beizhujiaoyu.com/uploads/202208/156127140430741.png" alt="" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/156127141383301.png" alt="" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/156127142264141.png" alt="" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/156127143085621.png" alt="" />

A 1 B 3 C 5 D 7

A <img src="https://www.beizhujiaoyu.com/uploads/202208/15612714725171.png" alt="" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/156127147947621.png" alt="" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/156127148744901.png" alt="" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/156127149398431.png" alt="" />

A <img src="https://www.beizhujiaoyu.com/uploads/202208/156127153388961.png" alt="" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/156127154183351.png" alt="" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/15612715533201.png" alt="" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/156127161230301.png" alt="" />

A -l B 0 C 1 D 2

A <img src="https://www.beizhujiaoyu.com/uploads/202208/156127167012381.png" alt="" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/156127167774781.png" alt="" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/156127168795901.png" alt="" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/156127169492671.png" alt="" />

A <img src="https://www.beizhujiaoyu.com/uploads/202208/156127179854201.png" alt="" /> B <img src="https://www.beizhujiaoyu.com/uploads/202208/156127180523321.png" alt="" /> C <img src="https://www.beizhujiaoyu.com/uploads/202208/156127181632291.png" alt="" /> D <img src="https://www.beizhujiaoyu.com/uploads/202208/156127182696041.png" alt="" />