Question

已知向量a=(cosa,sina),b =(cosb .sinb),且|a -b|=(2根号5)/5

Answer 1

a-b=((cosa-cosb),(sina-sinb))

|a-b|^2=(cosa-cosb)^2+(sina-sinb)^2=(2√5/5)^2=4/5

2-2cos(a-b)=4/5

cos(a-b)=3/5

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