cos(180-x)=-cosx
cos(75+α)=1/3 [ 75+α在第四象限]
cos［180-(75+α)］=-cos(75+α)=-1/3
∴cos(105-α)=-1/3
sin(75+α)=-√(1-1/9)=-2√2/3
sin［180-(75+α)］=-2√2/3
sin(105-α)=-2√2/3
sin(α-105)=2√2/3
∴cos(105-α)+sin(α-105)=-1/3+2√2/3＝(2√2-1)/3
tan315-sin(-840)+cos(-1050)
=tan(360-45)+sin(720+120)+cos(1050)
=tan(45)+sin(120)+cos(30)
=-1+√3/2+√3/2
=√3-1
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