Question

Find cos \theta θ if \sin\theta=-\frac{7}{15} sin ⁡ θ = − 7 15 and falls in quadrant 4

Answer 1

let's recall that on the IV Quadrant the x/cosine is positive and the y/sine is negative, and of course the hypotenuse is just a radius unit and therefore never negative.

`\bf sin(\theta )=\cfrac{\stackrel{opposite}{-7}}{\stackrel{hypotenuse}{15}}\impliedby \textit{let's find the \underline{adjacent side}} \\\\\\ \textit{using the pythagorean theorem} \\\\ c^2=a^2+b^2\implies √(c^2-b^2)=a \qquad \begin{cases} c=hypotenuse\\ a=adjacent\\ b=opposite\\ \end{cases}`

`\bf \pm√(15^2-(-7)^2)=a\implies \pm√(176)=a\implies \stackrel{\textit{IV Quadrant}}{+√(176)=a} \\\\[-0.35em] ~\dotfill\\\\ ~\hfill cos(\theta )=\cfrac{\stackrel{adjacent}{√(176)}}{\stackrel{hypotenuse}{15}}~\hfill`