cos80°*COS20°-COS10°COS110° =cos80°*COS20°+COS10°COS70° =cos80°*COS20°+sin10°sin20° =cos(80°-20°) =1/2
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cos80°*COS20°-COS10°*COS110° =sin10°*cos20°-cos10°*(-cos70°) =sin10°*cos20°+cos10°*sin20° =sin(10°+20°) =sin30° =1/2
cos80cos20-cos10cos110 本题中省去角度的单位"°". =cos80sin70-sin80(-cos70) =cos80sin70+sin80cos70 =sin(80+70) =sin150 =sin30 =1/2
cos80度乘以COS20度—COS10度COS110度 =cos80*cos20-sin(90-10)*cos110 =cos80*cos20+sin80*sin20 (cos110=-cos70=-sin20) =cos(80-20) =cos60 =1/2
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答：sin10°cos20°cos40° =sin（90°-10°）cos20°cos40° =coa80°cos20°cos40° =1/(8sin20°)*8s...详情>>
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