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“奇异偶同，象限定号”
cos(π/4-α)cos(π/4+α)
=cos(π/4-α)cos[-(π/4+α)]
=cos(π/4-α)sin[π/2-(π/4+α)]
=cos(π/4-α)sin(π/4-α)
=(1/2)*{sin[2*(π/4-α)]}
=(1/2)sin(π/2-2α)
=-(1/2)cos[(π/2-2α)-π/2]
=-(1/2)cos(2α)
cos(π/4-α)cos(π/4+α)
=cos(π/4-α)cos[-(π/4+α)]
=cos(π/4-α)sin[π/2-(π/4+α)]
=cos(π/4-α)sin(π/4-α)
=(1/2)*{sin[2*(π/4-α)]}
=(1/2)sin(π/2-2α)
=-(1/2)cos[(π/2-2α)-π/2]
=-(1/2)cos(2α)
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