Question

Solve this:


17. If $\cos \mathrm{x}=-0.78,\mathrm{where}\frac{\mathrm{\pi }}{2}<\mathrm{x}<\mathrm{\pi },\mathrm{what}\mathrm{is}\mathrm{the}\mathrm{value}\mathrm{of}\cos 2\mathrm{x}?$
(A) 0.22
(B) 0.37
(C) 0.43
(D) 0.50
(E) 0.67

Answer 1

Dear student
$$\begin{gathered} \mathrm{We}\mathrm{know}, \\ \cos 2\mathrm{x}=2{\cos }^2\mathrm{x}-1 \\ =2{\left(0.78\right)}^2-1 \\ =1.2168-1 \\ =0.2168 \\ \approx 0.22 \end{gathered}$$
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