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Question

use a trigonometric identity to find the indicated value in the specified quadrant. Function Value sin θ = - 3/5 Quadrant IV Value cos θ

Solution

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Answered 2 years ago
Answered 2 years ago
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Given

sinθ=35\sin \theta=-\frac{3}{5}

Quadrant IV We need to find the value of cosθ\cos \theta in the given quadrant.

Remember about the trigonometric identity

sin2θ+cos2θ=1\sin^2 \theta+\cos^2 \theta=1

Therefore,

sin2θ+cos2θ=1Subtract sin2θ from both sidescos2θ=1sin2θcosθ=±1sin2θIn the Quadrant IV : cosθ>0cosθ=1sin2θSubstitute sinθ=35cosθ=1(35)2cosθ=1925cosθ=25925cosθ=1625cosθ=45\begin{align*} \sin^2 \theta+\cos^2 \theta&=1 &&\text{Subtract $\sin^2 \theta$ from both sides}\\ \cos^2 \theta&=1-\sin^2 \theta\\ \cos \theta&=\pm \sqrt{1-\sin^2 \theta} &&\text{In the Quadrant IV : $\cos \theta>0$}\\ \cos \theta&= \sqrt{1-\sin^2 \theta} &&\text{Substitute $\sin \theta=-\frac{3}{5}$}\\ \cos \theta&=\sqrt{1-\left(-\frac{3}{5}\right)^2}\\ \cos \theta&=\sqrt{1-\frac{9}{25}}\\ \cos \theta&=\sqrt{\frac{25-9}{25}}\\ \cos \theta&=\sqrt{\frac{16}{25}}\\ \cos \theta&=\frac{4}{5} \end{align*}

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