Question

cos195*cos105+sin105*cos75=

Answer 1

$Cos195^0Cos105^0+Sin105^0Cos75^0=Cos(180^0+15^0)Cos(90^0+15^0)+Sin(90^0+15^0)Cos(90^0-15^0)=-Cos15^0(-Sin15^0)+Cos15^0Sin15^0=2Sin15^0Cos15^0=Sin30^0=frac{1}{2}$