a=126 sin126=sin(180-126)=sin54
a/sina=b/sinb=2r ( a-b)/(sin54-sin18)=2r
sin54-sin18
=2cos36sin18
=(2cos36sin18cos18)/cos18
=(sin36cos36)/cos18
=(2sin36cos36)/2cos18
=sin72/2cos18
=cos18/2cos18
=1/2得证
a=126 sin126=sin(180-126)=sin54
a/sina=b/sinb=2r ( a-b)/(sin54-sin18)=2r
sin54-sin18
=2cos36sin18
=(2cos36sin18cos18)/cos18
=(sin36cos36)/cos18
=(2sin36cos36)/2cos18
=sin72/2cos18
=cos18/2cos18
=1/2得证