u 1 =311sin(314 t +60 ° ) 、 u 2 =220 sin(314 t -30 ° ) ,两个电压的相位关系为
A.
u 1 超前 u 2 , 30 °
B.
u 1 超前 u 2 , 90 °
C.
u 2 超前 u 1 , 30 °
D.
u 2 超前 u 1 , 90
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