sin(-17Ï/5)=sin(-17Ï/5+4Ï)=sin(3Ï/5)=sin108度ï¼
sin108=cos18 (è§çåä½æ¯åº¦)
sin36=cos54
2sin18cos18=cos3*18
2sin18cos18=4cos^3(18)-3cos18
2sin18=4cos^2(18)-3
2sin18=4-4sin^2(18)-3
4sin^2(18)+2sin18-1=0
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sin18=(-2+2â5)/8=(â5-1)/4
cos18=â[1-(â5-1)^2/16]=â[1-(3-â5)/8=â[(5+â5)/8]
sin(-17Ï/5)=â[(5+â5)/8]
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