sin^2A+sin^2B+sin^2C-2cosAcosBcosC=2
sin^2A sin^2B sin^2C-2cosAcosBcosC=2
sina^2 sinb^2 sinc^2-2cosacosbcosc
=3-(cosa^2 cosb^2 cosc^2 2cosacosbcosc)
=3-{cosa*[cosa 2cosb*cosc] (1/2)*[cos(2b) cos(2c) 2]}
=3-{-cos(b c)*[-cos(b c) 2cosb*cosc] (1/2)*[cos(2b) cos(2c)] 1}
=3-{-cos(b c)*cos(b-c) cos(b c)*cos(b-c) 1}
=2
除了這種方法之外還有沒有別的方法?
sina^2 sinb^2 sinc^2-2cosacosbcosc
=3-(cosa^2 cosb^2 cosc^2 2cosacosbcosc)
=3-{cosa*[cosa 2cosb*cosc] (1/2)*[cos(2b) cos(2c) 2]}
=3-{-cos(b c)*[-cos(b c) 2cosb*cosc] (1/2)*[cos(2b) cos(2c)] 1}
=3-{-cos(b c)*cos(b-c) cos(b c)*cos(b-c) 1}
=2
除了這種方法之外還有沒有別的方法?
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