5、两向量a,b所在直线夹角为 π/4 lt 0, 那么下列说法正确的是 ()-|||-A、a,b夹角 dfrac (pi )(4) B、a,b夹角 dfrac (3pi )(4)-|||-C、a,b夹角可能 dfrac (3pi )(4) 或 dfrac (pi )(4) D.以上都不对

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