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sin10°sin30°sin50°sin70°

sin(90-)=cos 2sincos=sin2 ã sin10sin30sin50sin70 =cos80cos60cos40cos20 =cos80*1/2*cos40(cos20sin20)/sin20 =1/2*cos80cos40*1/2*sin40/sin20 =1/4*cos80*(cos40sin40)/sin20 =1/4*cos80*1/2*sin80/sin20 =1/8*cos80sin80/sin...

sin10sin50sin70 =sin10*1/2(cos20-cos120) =1/2sin10(cos20+1/2) =1/4(sin30-sin10)+1/4sin10 =1/4sin30 =1/8 sin10sin30sin50sin70 =1/2*1/8 =1/16

13sin10sin30sin50sin70⴮ʲô

sin10sin30sin50sin70=1/2sin(90-80)sin(90-40)sin(90-20) =1/2cos20Xcos40Xcos80 =1/2sin20Xcos20Xcos40Xcos80/sin20 =1/2X1/2Xsin40Xcos40Xcos80/sin20 =1/16Xsin160/sin20 sin160=sin20 =1/16

sin10=cos(90-10)=cos80 sin30=cos(90-30)=cos60 С90ȵĽa sina=cos90-a

ĸ ͬʱ cos10 sin10sin50sin70 = sin10sin50sin70 cos10/cos10 = sin10 cos10sin50sin70/cos10 = (1/2) sin20 sin50 sin70/cos10 = (1/2) sin20 sin50 cos20/cos10 = (1/4) sin40 sin50...

1.sin10*sin30*sin50*sin70 =cos10*sin10*sin30*sin50*sin70/cos10 =1/2*sin20*sin70*sin30*sin50/cos10 =1/2*sin20*cos20*sin30*sin50/cos10 =1/4sin40*sin50*sin30/cos10 =1/4sin40*cos40*sin30/cos10 =1/8sin80*sin30/cos10 =1/8cos10*sin3...

10=90-80

sin10o*sin30o*sin50o*sin70o =(1/2)*sin10o*sin50o*sin70o =(1/2)*2sin10o*cos10o*sin50o*sin70o/(2cos10o) =sin20o*sin50o*...

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