cos42°cos78°+sin42°cos168°怎么解

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cos42°cos78°+sin42°cos168°怎么解
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cos42°cos78°+sin42°cos168°怎么解
cos42°cos78°+sin42°cos168°怎么解

cos42°cos78°+sin42°cos168°怎么解
=cos42sin12-sin42cos12
=sin(12-42)
=sin-30=-0.5

=cos42cos78-sin42cos12
=cos42cos78-sin42sin78
=cos(78+42)
=cos120
=-1/2

cos168°=cos(180-12°)=-cos12°
cos42°cos78°+sin42°cos168
=cos42°sin12°-sin42°cos12°
=-sin30°
=-1/2

cos42°cos78°+sin42°cos168°
=cos42°cos78°-sin42°cos12°
=cos42°cos78°-sin42°sin78°
=cos(42°+78°)
=cos120°
=-1/2;

-0.5

-1/2