Ta có
$\cos(225) = \cos(360 - 135) = \cos(-135) = \cos(45 - 180) = \cos(180 - 45) = - \cos 45 = -\dfrac{\sqrt{2}}{2}$
$\sin(240) = \sin(360-120) = \sin(-120) = -\dfrac{\sqrt{3}}{2}$
$cot(-15) = \dfrac{\cos(-15)}{\sin(-15)} = \dfrac{\cos(15)}{-\sin(15)} = -cot(15)=-2 - \sqrt{3}$
$\tan(75) = cot(15) = 2 + \sqrt{3}$.