4cos(a/4)*cos(π/2+a/4)*cos(π+a/2)=4cos(a/4)*(-sin(a/4))*(-cos(a/2))=
=-2sin(a/2)*(-cos(a/2)=sina

1:(1-sina/cosa)-1:(1+sina/cosa)=cosa/(cosa-sina)-cosa/(cosa+sina)=
=cosa(cosa+sina-cosa+sina)/(cos²a-sin²a)=cosa*2sina/cos2a=
=sin2a/cos2a=tg2a