Cos75cos195-8cos20cos40cos80

Question

Cos75cos195-8cos20cos40cos80

Answer 1

Cos75cos195=cos(90-15)*cos(180+15)=sin15*(-cos15)=
=-1/2*sin30=-1/2*1/2=-1/4
8cos20cos40cos80=8sin20cos20cos40cos80/sin20=4sin40cos40cos80/sin20=
=2sin80cos80/sin20=sin160/sin20=sin20/sin20=1
-1/4-1=-1,25