(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