下列正弦量的相量表达式分别为：i1=3sinωt ,i2 =4sin(ωt-150°),i3=- 5sin(ωt- 60°)i4 =6cos(ωt+30°).
A. 3∠0°；4∠-150°；5∠120°；6∠120°
B. 3∠0°；4∠150°；5∠120°；6∠120°
C. 3∠0°；4∠-150°；5∠-60°；6∠120°
D. 3∠0°；4∠-150°；5∠120°；6∠30°
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一个电阻R=15Ω、电感L=12mH的线圈与C=5uF的电容器相串联,接在电压u =100sinωt V的电源上，ω=5000rad/s。试求电流i、电容器端电压uc.和线圈端电压uw 。
A. i= 4 sin(ωt+53.10°) AuC =160 sin(ωt -143.1°) VuC =248sin(ωt +22.9°) V
B. i= 4 sin(ωt-53.10°) AuC =160 sin(ωt -143.1°) VuC =248sin(ωt -22.9°) V
C. i= 4 sin(ωt-53.10°) AuC =160 sin(ωt -143.1°) VuC =248sin(ωt +22.9°) V
D. i= 4 sin(ωt-53.10°) AuC =160 sin(ωt +143.1°) VuC =248sin(ωt +22.9°) V
What has replaced the era when people could safely keep their front door unlocked?It has been replaced by an era _______________.
A. when people can afford a lot of security devices
B. when people emloy various security devices at home
C. when people attach importance to their life quality
D. when people are keen to purchase insurance
Why is it common to see small notices pasted on the windows of the most pleasant of homes?
A. Because people intend to make a fair profit on the sale of their house.
Because people want to show their house isprotected by some security agency.
C. Because people try to create an atmosphere of pleasure and elegance.
D. Because people aim to warn strangers to keep away from their house.
Which of the following is included in the security devices?
A. Dead-bolt locks and security chains.
B. Electronic alarm systems and trip wires.
C. Public-service advertisements launched by an insurance company.
Dead-bolt locks andtrip wires.
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