Obliczamy wartości danych wyrażeń. Korzystamy z podstawowych wzorów redukcyjnych zapisanych na stronie 179.

$\text{a)}\cos {120}^{\circ }=\cos \left({180}^{\circ }-{60}^{\circ }\right)=-\cos {60}^{\circ }=-\frac{1}{2}$  

$\cos {135}^{\circ }=\cos \left({180}^{\circ }-{45}^{\circ }\right)=-\cos {45}^{\circ }=-\sqrt{/}2$  

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$\text{b)}\cos {210}^{\circ }=\cos \left({180}^{\circ }+{30}^{\circ }\right)=-\cos {30}^{\circ }=-\frac{\sqrt{3}}{2}$  

$\cos {225}^{\circ }=\cos \left({180}^{\circ }+{45}^{\circ }\right)=-\cos {45}^{\circ }=-\frac{\sqrt{2}}{2}$  

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$\text{c)}\cos \left(-{12}^{\circ }\right)=\cos {12}^{\circ }\approx 0,98$  

$\cos \left(-{61}^{\circ }\right)=\cos {61}^{\circ }\approx 0,48$  

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$\text{d)}\cos \left(-{280}^{\circ }\right)=\cos {280}^{\circ }=\cos \left(-{80}^{\circ }+{360}^{\circ }\right)=\cos \left(-{80}^{\circ }\right)=\cos {80}^{\circ }\approx 0,17$  

$\cos \left(-{315}^{\circ }\right)=\cos {315}^{\circ }=\cos \left(-{45}^{\circ }+{360}^{\circ }\right)=\cos \left(-{45}^{\circ }\right)=\cos {45}^{\circ }=\frac{\sqrt{2}}{2}$  

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$\text{e)}\cos {192}^{\circ }=\cos \left({180}^{\circ }+{12}^{\circ }\right)=\cos {12}^{\circ }\approx -0,98$  

$\cos {203}^{\circ }=\cos \left({180}^{\circ }+{23}^{\circ }\right)=\cos {23}^{\circ }\approx -0,92$  

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$\text{f)}\cos \left(-{124}^{\circ }\right)=\cos {124}^{\circ }=\cos \left({180}^{\circ }-{56}^{\circ }\right)=-\cos {56}^{\circ }\approx -0,56$  

$\cos \left(-{155}^{\circ }\right)=\cos {124}^{\circ }=\cos \left({180}^{\circ }-{25}^{\circ }\right)=-\cos {25}^{\circ }\approx -0,91$