求sin²10°+cos²40°+sin10°cos40°的值
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求sin²10°+cos²40°+sin10°cos40°的值
求sin²10°+cos²40°+sin10°cos40°的值
求sin²10°+cos²40°+sin10°cos40°的值
sin²10°+cos²40°+sin10°cos40°
=(1-cos20º)/2+(1+cos80º)/2+sin10°cos40°
=1+1/2(cos80º-cos20º)+sin10°cos40°
=1-sin50ºsin30º+1/2(sin50º-sin30º) (前面和差化积,后面积化和差)
=1-1/4=3/4