搜题请搜索小程序“爱下问搜题”
问题 单项选择题

建立E-R模型的工作，属于软件生命周期中的( )。

A．需求分析阶段

B．设计阶段

C．编码阶段

D．测试阶段

答案

参考答案：B

解析： 建立E-R模型是数据库概念设计的重要内容，而概念设计是设计阶段的组成部分。

单项选择题

System analysis is traditionally done top-down using structured analysis based on （）. Object-oriented analysis focuses on creation of models. The three types of the analysis model are （）. There are two substages of object-oriented analysis.（）focuses on real-world things whose semantics the application captures. The object constructed in the requirement analysis shows the （） of the real-world system and organizes it into workable pieces.（） addresses the computer aspects of the application that are visible to users. The objects are those which can be expected to vary from time to time quite rapidly.

System analysis is traditionally done top-down using structured analysis based on （）.

A. functional decomposition

B. object abstraction

C. data inheritance

D. information generalization
查看答案
问答题

一台三相变压器，额定频率fN＝50Hz，额定容量SN＝1000kV·A，联结组标号为Yd11。在低压侧加额定电压U2N＝3.15kV，测得高压侧空载电压U10＝10kV。在高压侧做短路试验，测得额定电流下的短路电压Uk＝550V，短路损耗pkN＝10kW。不考虑温度换算。

（1）若在低压侧做短路试验，则测得的短路阻抗为多少欧姆？

（2）求该变压器的短路电阻、短路电抗的标幺值；

（3）当该变压器高压侧接在额定电压、频率为60Hz的电网上，带功率因数cosϕ2＝0.8（超前）的额定负载运行时，低压侧电压U2是多少？
查看答案