Question

cos(15°) / (cos(45°) – cos(105°))

Answer 1

cos105=cos(180-75)=- cos75 - по формулам привидения
cos45-(-cos75)= cos45+cos75=2cos((45+75)/2)*cos((45-75)/2)=
=2cos60*cos(-15)=2cos15*cos60
косинус  - функция четная; cos(-α)=cosα
cos15/(2cos15*cos60)=1/(2cos60)=1/(2*1/2)=1