cos51°cos9°-cos39°sin9°=
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cos51°cos9°-cos39°sin9°=cos51°cos9°-cos39°sin9°=cos51°cos9°-cos39°sin9°=cos51°cos9°-cos39°sin9°=cos5
cos51°cos9°-cos39°sin9°=
cos51°cos9°-cos39°sin9°=
cos51°cos9°-cos39°sin9°=
cos51°cos9°-cos39°sin9°
=cos51°cos9°-sin51°sin9°
=cos(51°+9°)
=cos60°
=1/2
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cos51°cos9°-cos39°sin9°=sin(90°-51°)cos9°-cos39°sin9°=sin31°cos9°-cos39°sin9°=sin(39°-9°)=sin30°=1/2;
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