Можно записать так 15 - 6 = 9
14-5 = 9
13-4 = 9
12-3 = 9
11-2 = 9
10-1=9
(2cos54+3cos36-2cos144)/(sin70sin74-sin20sin16)=
=(2cos(90-36)+3cos36-2cos(180-36))/(sin(90-20)sin(90-16)-sin20sin16)=
=(2sin36+3cos36+2cos36)/(cos20cos16-sin20sin16)=(2sin36+5cos36)/cos(20+16)=
=(2sin36+5cos36)/cos36=2tg36+5