已知cosθ等於負5分之3,且θ屬於(π,2分之3π),求cos2分之θ的值

已知cosθ等於負5分之3,且θ屬於(π,2分之3π),求cos2分之θ的值


cosθ=-3/5,θ∈(π,3π/2),則有θ/2∈(π/2,3π/4),則有cos(θ/2)<0.
∵cosθ=2cos^2(θ/2)-1,
∴有cos(θ/2)=-√[(1+cosθ)/2]
=-√[(1-3/5)/2]
=-2√5/5.



cos(45°—30°)等於多少


cos(45°—30°)
=cos(45)cos(30)+sin(45)sin(30)
=(√6+√2)/4



cos(45°-α)等於什麼,是cosα?還是什麼?
其實這道題是這樣的求2sin14°cos31°+sin17°等於多少,


2sin14°cos31°+sin17°=2sin14(cos14cos17-sin14sin17)+sin17=2sin14cos14cos17-2sin14sin14sin17+sin17=sin28cos17+sin17(1-2sin14sin14)=sin28cos17+sin17cos28=sin45=根號2/2