cosa*cos2a*cos4a*.*cos2^(n-1)a
=sina*cosa*cos2a*....*cos2^(n-1)a/sina
=1/2sin2a*cos2a*cos4* ...* cos2^(n-1)a/sina
=1/4sin4a*cos4a*...*cos2^(n-1)a/sina
=1/8sin8a*...*cos2^(n-1)a/sina
=1/2^(n-1)sin2^(n-1)*cos2^(n-1)a/sina
=1/2^n*sin2^na/sina