Korzystamy ze wzorów:
[tex]\text{tg }\alpha=\text{tg }(\alpha-180^{\circ})\\\\\cos\alpha=-\cos(180^{\circ}-\alpha)\\\\\text{tg }(-\alpha)=-\text{tg }\alpha\\\\\cos(-\alpha)=\cos\alpha[/tex]
Mamy:
[tex]\text{tg }150^{\circ}=\text{tg }(-30^{\circ})=-\text{tg }30^{\circ}=-\dfrac{\sqrt{3}}{3}\\\\\cos240^{\circ}=-\cos(180^{\circ}-240^{\circ})=-\cos(-60^{\circ})=-\cos60^{\circ}=-\dfrac{1}{2}[/tex]
Więc:
[tex]3\text{tg }150^{\circ}-2\cos240^{\circ}=3\cdot\Big(-\dfrac{\sqrt{3}}{3}\Big)-2\cdot\Big(-\dfrac{1}{2}\Big)=-\sqrt{3}+1\approx-0,732[/tex]