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Calculate each of the following without the use of a calculator:

(a) $\cos 80^{\circ} \cdot \cos 10^{\circ}-\sin 80^{\circ} \cdot \sin 10^{\circ}$

(b) $\cos 70^{\circ} \cdot \cos 40^{\circ}+\sin 70^{\circ} \cdot \sin 140^{\circ}$

(c) $\sin 280^{\circ} \cdot \cos 160^{\circ}-\cos 100^{\circ} \cdot \sin 200^{\circ}$

(d) $\cos 265^{\circ} \cdot \sin 355^{\circ}-\sin 85^{\circ} \cdot \cos 175^{\circ}$

(e) $\cos 65^{\circ} \cdot \cos 295^{\circ}-\sin 115^{\circ} \cdot \cos 205^{\circ}$

(f) $\cos 50^{\circ} \cdot \sin 260^{\circ}+\cos 10^{\circ} \cdot \sin 140^{\circ}$
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