What is the value of cos [(180 – θ)/2] cos [(180 – 9θ)/2] + sin [(180 – 3θ)/2] sin [(180 – 13θ)/2]?

• sin 2θ sin 4θ
• cos 2θ cos 6θ
• sin 2θ sin 6θ
• cos 2θ cos 4θ