cos100cos120cos140cos160
=cos(180-80)cos(180-60)cos(180-40)cos(180-20)
=(-cos80)(-cos60)(-cos40)(-cos20)
=cos20xxcos40xxcos80cos60
=1/(8sin20)xx4*2sin20xxcos20xxcos40xxcos80cos60
=1/(8sin20)xx2*2sin40xxcos40xxcos80cos60
=1/(8sin20)xx2sin80xxcos80cos60
=1/(8sin20)xxsin160cos60
=1/(8sin20)xxsin(180-20)xx1/2
=1/(16sin20)xxsin20=1/16