3sin2a-2sinacosa
=6sinacosa-2sinacosa
=4sinacosa
=2sin2a
if sina=2cos2 => cosa=sqrt(1-4cos2)带入即可
if sina=2cosa, sina^2+cosa^2=1 =>cosa=1/sqrt(5),sina=2/sqrt(5)

then 2sin2a=2*2*1/sqrt(5)*2/sqrt(5)=8/5 3sin2a-2sinacosa
=6sinacosa-2sinacosa
=4sinacosa
=2sin2a
if sina=2cos2 => cosa=sqrt(1-4cos2)带入即可
if sina=2cosa, sina^2+cosa^2=1 =>cosa=1/sqrt(5),sina=2/sqrt(5)

then 2sin2a=2*2*1/sqrt(5)*2/sqrt(5)=8/5