Prove the following:
(a) \(\cos (\alpha+\beta)+\cos (\alpha-\beta)=2 \cos \alpha \cdot \cos \beta\)
(b) \(\sin \left(\theta+30^{\circ}\right)-\sin \left(\theta-30^{\circ}\right)=\cos \theta\)
(c) \(\cos \left(45^{\circ}-\theta\right)-\sin \left(45^{\circ}-\theta\right)=\sqrt{2} \sin \theta\)
(d) \(\sqrt{3} \sin \left(\theta+60^{\circ}\right)-\sin \left(\theta+30^{\circ}\right)=\cos \theta\)
(e) \(\sin \left(\mathrm{A}-60^{\circ}\right)+\cos \left(\mathrm{A}-30^{\circ}\right)=\sin \mathrm{A}\)