prove the identity cos 3 - cos7 / sin 7 +sin3 =tan2 Solution The given cos 3 - cos7 / sin 7 +sin3 =tan2 Let LHS= cos 3 - cos7 / sin 7 +sin3 {-2sin((10)/2)sin((-4)/2)}/2sin((10)/2)cos((4)/2) (since cos c-cos d=-2sin((c+d)/2)sin((c-d)/2) and sin c +sin d=2sin(c+d)/2 cos(c-d)/2) LHS={-2sin((10)/2)sin((-4)/2)}/2sin((10)/2)cos((4)/2) =-sin((-4)/2)/cos((4)/2) by simplification =sin2/cos2 (since sin(-2)=-sin2) =tan2 therefore LHS=RHS.