Karen Simpson

2022-01-14

$\mathrm{cos}5=\sqrt{1-{a}^{2}}$ which statement is true?
1) $\mathrm{sin}20=4a\sqrt{1-{a}^{2}}\sqrt{1-4{a}^{2}}$
2) $\mathrm{tan}50=\frac{1-a}{1+a}$
3) $\mathrm{tan}10=\frac{2}{a\sqrt{1-{a}^{2}}}$
4) $\mathrm{cos}65=\frac{12}{\sqrt{1-{a}^{2}}-\sqrt{3}a}$

Barbara Meeker

Expert

Hint: Assuming the arguments are in degrees:
$\mathrm{cos}\left({65}^{\circ }\right)=\mathrm{cos}\left({60}^{\circ }+{5}^{\circ }\right)=\mathrm{cos}\left({60}^{\circ }\right)\mathrm{cos}\left({5}^{\circ }\right)-\mathrm{sin}\left({60}^{\circ }\right)\mathrm{sin}\left({5}^{\circ }\right)$

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