在△
ABC和△
AEF中,
B是
EF的中點,
AB=
EF=1,
BC=6,
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,若

,則

與

的夾角的余弦值等于
。
因為

,所以

,即

。因為

,

,

,所以

,即

。設(shè)

與

的夾角為
θ,則有

,即3cos
θ=2,所以

。
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