已知sin(3π-α)=-2sin(π/2+α)则sinαcosα

已知sin(3π-α)=-2sin(π/2+α)则sinαcosα
已知sin(3π-α)=-2sin(π/2+α)则sinαcosα

已知sin(3π-α)=-2sin(π/2+α)则sinαcosα
用a代替α吧
sin(3π-a)=-2sin(π/2+a)
sina=-2cosa
tana=-2
sinacosa
=sinacosa/(sin²a+cos²a)
分子分母同时除以cos²a
=tana/(tan²a+1)
=-2/(4+1)
=-2/5

-2/5