Find all solutions to this equation please

Mathematics

2 answers:

valentina_108 [34]3 years ago

6 0

Answer: $\bold{d)\ \dfrac{5}{8},\ \dfrac{35}{8},\ \dfrac{45}{8},\ \dfrac{75}{8}}$

Step-by-step explanation:

$20\cos\bigg(\dfrac{2\pi}{5}x\bigg)=10\sqrt2\\\\\\\cos\bigg(\dfrac{2\pi}{5}x\bigg)=\dfrac{\sqrt2}{2}\\\\\\\cos^{-1}\bigg[\cos\bigg(\dfrac{2\pi}{5}x\bigg)\bigg]=\cos^{-1}\bigg(\dfrac{\sqrt2}{2}\bigg)\\\\\\ \dfrac{2\pi}{5}x=\cos^{-1}\dfrac{\sqrt2}{2}\\\\\\\text{Where on the Unit Circle is cos}=\dfrac{\sqrt2}{2}\quad \rightarrow\quad at\ \dfrac{\pi}{4}\ and\ \dfrac{7\pi}{4}\\\\\\\text{All possible solutions are:}\ \dfrac{\pi}{4}+2\pi n\quad and\quad \dfrac{7\pi}{4}+2\pi n\\\\\text{:n = number of rotations}$

$\dfrac{2\pi}{5}x=\dfrac{\pi}{4}\quad \longrightarrow \quad \bigg(\dfrac{5}{2\pi}\bigg)\dfrac{2\pi}{5}x=\dfrac{\pi}{4}\bigg(\dfrac{5}{2\pi}\bigg)\quad \longrightarrow \quad \large\boxed{x=\dfrac{5}{8}}}\\\\\\ \dfrac{2\pi}{5}x=\dfrac{7\pi}{4}\quad \longrightarrow \quad \bigg(\dfrac{5}{2\pi}\bigg)\dfrac{2\pi}{5}x=\dfrac{7\pi}{4}\bigg(\dfrac{5}{2\pi}\bigg)\quad \longrightarrow \quad \large\boxed{x=\dfrac{35}{8}}}\\$

$\dfrac{2\pi}{5}x=\dfrac{\pi}{4}+2\pi\quad \longrightarrow \quad \bigg(\dfrac{5}{2\pi}\bigg)\dfrac{2\pi}{5}x=\dfrac{9\pi}{4}\bigg(\dfrac{5}{2\pi}\bigg)\quad \longrightarrow \quad \large\boxed{x=\dfrac{45}{8}}}\\\\\\ \dfrac{2\pi}{5}x=\dfrac{7\pi}{4}+2\pi\quad \longrightarrow \quad \bigg(\dfrac{5}{2\pi}\bigg)\dfrac{2\pi}{5}x=\dfrac{15\pi}{4}\bigg(\dfrac{5}{2\pi}\bigg)\quad \longrightarrow \quad \large\boxed{x=\dfrac{75}{8}}}\\$