Chứng minh các đẳng thức lượng giác sau:
$a)\quad\tan9^{\circ}-\tan27^{\circ}-\tan63^{\circ}+\tan81^{\circ}=4.$
$b)\quad\tan20^{\circ}-\tan40^{\circ}+\tan80^{\circ}=3\sqrt3.$
$c)\quad \tan10^{\circ}-\tan50^{\circ}+\tan60^{\circ}+\tan70^{\circ}=2\sqrt3.$
$d)\quad \tan30^{\circ}+\tan40^{\circ}+\tan50^{\circ}+\tan60^{\circ}=\frac{8\sqrt3}{3}\cdot\cos20^{\circ}.$
