(8sin^4 3п/20 - 8 cos^4 3п20) / sin 4п/5

8(sin^2 (3pi/20)+cos^2 (3pi/20))(sin^2 (3pi/20)-cos^2 (3pi/20))

---------------------------------------------------------------------------------- =

                         sin 4pi/5

-8cos(2*3pi/20)       8cos 3pi/10

-------------------- = - ----------------- = -8

sin(pi/2+3pi/10)        cos 3pi/10