cotθ=cosθ/sinθ=3
cosθ=3sinθ
(cosθ)^2=9(sinθ)^2
1-(sinθ)^2=9(sinθ)^2
(sinθ)^2=1/10
cos2θ-(1/2)sin2θ
=1-2(sinθ)^2-sinθcosθ
=1-2(sinθ)^2-3(sinθ)^2
=1-5(sinθ)^2
=1-5*1/10
=1/2