问题
选择题
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答案
因为sin(-250°)cos70° cos2155°-sin225°
=-sin250°cos70° cos225°-sin225°
=-sin(180°+70°)cos70° cos225°-sin225°
=sin70°cos70° cos 50°
=sin140° 2cos 50°
=sin40° 2cos 50°
=cos50° 2cos 50°
=
．1 2
故选C．
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