如图，平面回路上有一段ab长为L ,均匀磁场B与回路平面垂直，B随时间作如下变化：[img=91x22]1802f94aa24db98.png[/img]。ab到左边的距离为x，求感应电动势。[img=424x310]1802f94ab760377.png[/img] A: [img=111x22]1802f94ac0150b9.png[/img] B: [img=100x22]1802f94ac909b3a.png[/img] C: [img=37x19]1802f94ad255091.png[/img] D: 0

如图所示，圆形截面区域内存在着与截面相垂直的磁场，磁感应强度随时间变化．磁场区域外有一与圆形截面共面的矩形导体回路 <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/0eea13cd51b54af3b701a0035e6416fe.png" />，以<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/865048ee310d46e09b743d6cfc93fa4a.png" />表示在导体<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/55379797515a41b88e0dcb947c44b585.png" />段上产生的感生电动势， I 表示回路中的感应电流，则（ ）<img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/95d31735160f47dcb4d8f5ac973c0ce8.png" /> A: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/986a62c34302465cad899f00080acc4f.png" /> B: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/4b6ab53aeebc4f92b722d8fff6237a87.png" /> C: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/d869de90826645c1b386e2019aa86be2.png" /> D: <img src="http://image.zhihuishu.com/zhs/onlineexam/ueditor/201804/da8edb3a9af0442bb11dcee16f061375.png" />

设投掷1颗骰子的点数为X,则( ) 未知类型：{'options': ['E(X)=3.5,D(X)= [img=29x21]17e43c3db9dc21b.png[/img]', ' E(X)=3.5, D(X)= [img=22x41]17e43c3dc2146ee.png[/img]', ' E(X)=3.5,D(X)=3.5', ' E(X)=3.5,D(X)=[img=22x41]17e43c3dcacf7d1.png[/img]'], 'type': 102}

已知随机变量X的分布列如下：[img=386x130]17e43ec4c459e73.png[/img]，则E(X)= A: 17/30 B: m未知，无法求出 C: -30/17 D: -17/30

对于两个随机变量X与Y，若E(XY)=E(X)E(Y)，则（ ）. 未知类型：{'options': ['', ' [img=347x54]17e0b3b40ed05ed.png[/img]', ' [img=215x48]17e0b3b429afcab.png[/img]', ' [img=244x46]17e0b3b43fd7aa9.png[/img]'], 'type': 102}