梦山桥
幼苗
共回答了15个问题采纳率:80% 举报
1)cos105 = cos(90+15)=-sin15
cos30=1-2sin15^2 = √3 /2, sin15^2= (2-√3)/4, sin15=√(2-√3) /2
cos105=-√(2-√3) /2
2)sin165=sin(180-15)=sin15=√(2-√3) /2
3) cos(α+β)cosβ+sin(α+β)sinβ = cos(α+β-β)=cosα
4) sin(θ+105°)cos(θ-15°)-cos(θ+105°)sin(θ-15°)=sin(θ+105°-(θ-15°))=sin120°=√3/2
5) cos(π/4+α)+sin(π/4+α)=√2 ( √2/2cos(π/4+α)+ √2/2sin(π/4+α)= √2[sinπ/4cos(π/4+α)+ cosπ/4sin(π/4+α)]=√2sin(π/4+π/4+α)
=√2sin(π/2+α)=√2cosα
1年前
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