深蓝GG
幼苗
共回答了21个问题采纳率:90.5% 举报
因为cosA=cos-A 和 sin2A=2sinAcosA
所以 sin(105°-α)cos(α-105°)
=0.5sin(210°-2α)
=0.5sin(180°-(210°-2α))
=0.5sin(2α-30°)
=0.5sin(2(α-15°))
=sin(α-15°)cos(α-15°) (因sinA=-cos(90°+A) cosA=sin(90°+A))
=-cos(75°+α)sin(75°+α) (A在第三象限,所以75°+α的sin值为负,sin(75°+α)=-2squr(2)/3 squr为根号符)
=2squr(2)/9
1年前
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