Find exact value of \cos (\frac{9\pi}{4}) How do you get (\frac

facas9 2021-08-13 Answered
Find exact value of \(\displaystyle{\cos{{\left({\frac{{{9}\pi}}{{{4}}}}\right)}}}\)
How do you get \(\displaystyle{\left({\frac{{\pi}}{{{4}}}}+{2}\pi\right)}\)

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Expert Answer

wheezym
Answered 2021-08-14 Author has 13806 answers
\(\displaystyle{\frac{{{9}\pi}}{{{4}}}}={\frac{{\pi}}{{{4}}}}+{\frac{{{8}\pi}}{{{4}}}}={\frac{{\pi}}{{{4}}}}+{2}\pi\)
so \(\displaystyle{\frac{{\pi}}{{{4}}}}\) is the reference angle \(\displaystyle\angle{45}^{{{o}}}\)
\(\displaystyle{\cos{{\left({\frac{{\pi}}{{{4}}}}\right)}}}={\frac{{\sqrt{{{2}}}}}{{{2}}}}\)
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