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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...

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*sin30/co...

10=90-80

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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...

sin10=cos80 cos20=sin70 sin30=cos60 cos40=sin50 sin90=1 sin10cos20sin30cos40sin50cos60sin70cos80sin90 =(sin10cos20sin30cos40)^2 =...

S=(1/2)sin10sin50sin70=(1/2)[cos80cos40cos20] cos20cos40cos80 ====>>> ӷĸͬ8sin20㣬ʹsin2a=2sinacosa 𰸣S=1/16

9/8//Ϊ˷㣬ʡԡ//ǹʽsin3=3sin-4sin³//Σ4sin³=3sin-sin34(sin⁴10+sin⁴50+sin⁴70)=sin10(3sin10-sin30)+sin50(3sin50-sin150)+sin70(3sin70...

Ϊ1/2, 4sin10sin50sin70 =4cos80cos40cos20 =4cos80cos40cos20sin20/sin20 =2cos80cos40sin40/sin20 =sin80cos80/sin20 =2sin80cos80/2sin20 =sin160/2sin20 =1/2 ʵֹһַк֣ܶǵøҼӷְлл1

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