cos(π-2α)为什么等于-cos2α

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cos(α+π/4)为什么等于根号2/2cosα-根号2/2sinα?
cosα+sinα不等于 A.√2cos[α-(π/4)] B.√2cos[(π/4)-α] C.cos[α-(π/4)
sin(π/2+α)·cos(π/2-α)/cos(π+α)+sin（π-α）·cos（π/2+α）/sin（π+α）=
sin(3π+α )=1/4,求cos(π+α)/cosα[cos(π+α)-1]+cos(α-2π)/cos(α+2π
已知tanα=2,则cos（2α+π）等于?和sin（π-α）cos（-α）=?
化简[sin(π/2+α)*cos(π/2-α)]/cos(π+α)+[sin(π-α)*cos(π/2+α)]/sin
化简cos(π+α)cos(π2+α)cos(11π2−α)cos(π−α)sin(−π−α)sin(9π2+α)的结果
（1）化简：sin(2π−α)cos(π+α)cos(3π2+α)cos(13π2−α)cos(π−α)sin(3π−α
化简[sin(π/2-α）cos(π/2-α)]/cos(π+α)-[sin(π-α)cos(π/2-α)]/sin(π
设f(α)=[2sin(π+α)cos(π-α)-cos(π+α)]/[(1+sin^2α+sin(π-α)-cos^2