第9题求过程 10
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sin10sin50sin70
=cos80cos40cos20
=cos80cos40cos20sin20 / sin20
=(1/2)cos80cos40sin40 / sin20
=(/4)cos80sin80 / sin20
=(1/8)sin160 / sin20
=1/8--------------------------cos10cos50cos70＝0.5(cos60+cos40)cos70
=0.5(0.5cos70+cos70cos40)
=0.5[0.5cos70+0.5(cos110+cos30)]
=0.5(0.5cos70-0.5cos70+0.5cos30)=0.25cos30=√3/8-------------sin10'sin50'sin70'+cos10'cos50'cos70' =1/8+√3/8
=cos80cos40cos20
=cos80cos40cos20sin20 / sin20
=(1/2)cos80cos40sin40 / sin20
=(/4)cos80sin80 / sin20
=(1/8)sin160 / sin20
=1/8--------------------------cos10cos50cos70＝0.5(cos60+cos40)cos70
=0.5(0.5cos70+cos70cos40)
=0.5[0.5cos70+0.5(cos110+cos30)]
=0.5(0.5cos70-0.5cos70+0.5cos30)=0.25cos30=√3/8-------------sin10'sin50'sin70'+cos10'cos50'cos70' =1/8+√3/8
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