3.2.1: Trig Identities to Find Exact Trigonometric Values
- Page ID
- 4228
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Using Trig Identities to Find Exact Trig Values
You are given the following information about \(\theta\)
\(\sin\theta =\dfrac{2}{3}\), \(\dfrac{\pi}{2}<\theta <\pi\)
What are \(\cos\theta\) and \(\tan\theta \)?
Trigonometric Identities
You can use the Pythagorean, Tangent and Reciprocal Identities to find all six trigonometric values for certain angles. Let’s walk through a few problems so that you understand how to do this.
Let's solve the following problems using trigonometric identities.
- Given that \(\cos\theta =\dfrac{3}{5}\) and \(0<\theta <\dfrac{\pi}{2}\), find \(\sin \theta\).
Use the Pythagorean Identity to find sin\theta .
\(\begin{aligned}
\sin ^{2} \theta+\cos ^{2} \theta &=1 \\
\sin ^{2} \theta+\left(\dfrac{3}{5}\right)^{2} &=1 \\
\sin ^{2} \theta &=1-\dfrac{9}{25} \\
\sin ^{2} \theta &=\dfrac{16}{25} \\
\sin \theta &=\pm \dfrac{4}{5}
\end{aligned}\)
Because \(\theta\) is in the first quadrant, we know that sine will be positive. \(\sin\theta =\dfrac{4}{5}\)
- Find \(\tan\theta \) from #1 above.
Use the Tangent Identity to find \(\tan\theta\).
\(\tan\theta =\dfrac{\sin\theta}{\cos\theta}=\dfrac{\dfrac{4}{5}}{\dfrac{3}{5}}=\dfrac{4}{3}\)
- Find the other three trigonometric functions of \(\theta\) from #1.
To find secant, cosecant, and cotangent use the Reciprocal Identities.
\(\csc \theta=\dfrac{1}{\sin \theta}=\dfrac{1}{\dfrac{4}{5}}=\dfrac{5}{4} \quad \sec \theta=\dfrac{1}{\cos \theta}=\dfrac{1}{\dfrac{3}{5}}=\dfrac{5}{3} \quad \cot \theta=\dfrac{1}{\tan \theta}=\dfrac{1}{\dfrac{4}{3}}=\dfrac{3}{4}\)
Earlier, you were asked to find \(\cos\theta\) and \(\tan\theta\) of \(\sin\theta =\dfrac{2}{3}\), \(\dfrac{\pi}{2}<\theta <\pi \).
Solution
First, use the Pythagorean Identity to find \(\cos\theta\).
\(\begin{aligned}
\sin ^{2} \theta+\cos ^{2} \theta &=1 \\
\left(\dfrac{2}{3}\right)^{2}+\cos ^{2} \theta&=1 \\
\cos ^{2} \theta &=1-\dfrac{4}{9} \\
\cos ^{2} \theta &=\dfrac{5}{9} \\
\cos \theta &=\pm \dfrac{\sqrt{5}}{3}
\end{aligned}\)
However, because \(\theta\) is restricted to the second quadrant, the cosine must be negative. Therefore, \(\cos \theta =−\dfrac{\sqrt{5}}{3}\).
Now use the Tangent Identity to find tan\theta .
\(\tan\theta =\dfrac{\sin\theta}{\cos\theta}=\dfrac{\dfrac{2}{3}}{−\dfrac{\sqrt{5}}{3}}=−2\sqrt{5}=\dfrac{−2\sqrt{5}}{5}\)
Find the values of the other five trigonometric functions.
\(\tan\theta =−\dfrac{5}{12}\), \(\dfrac{\pi}{ 2}<\theta <\pi\)
Solution
First, we know that \theta is in the second quadrant, making sine positive and cosine negative. For this problem, we will use the Pythagorean Identity \(1+\tan^2\theta =\sec^2\theta\) to find secant.
\(\begin{aligned}
1+\left(-\dfrac{5}{12}\right)^{2} &=\sec ^{2} \theta \\
1+\dfrac{25}{144} &=\sec ^{2} \theta \\
\dfrac{169}{144} &=\sec ^{2} \theta \\
\pm \dfrac{13}{12} &=\sec \theta \\
-\dfrac{13}{12} &=\sec \theta
\end{aligned}\)
If \(\sec\theta =−\dfrac{13}{12}\), then \(\cos\theta =−\dfrac{12}{13}\). \(\sin\theta =\dfrac{5}{13}\) because the numerator value of tangent is the sine and it has the same denominator value as cosine. \(\csc\theta =\dfrac{13}{5}\) and \(\cot\theta =−\dfrac{12}{5}\) from the Reciprocal Identities.
\(\csc \theta =−8\), \(\pi <\theta <\dfrac{3 \pi}{2}\)
Solution
\theta is in the third quadrant, so both sine and cosine are negative. The reciprocal of \(\csc\theta =−8\), will give us \(\sin\theta =−\dfrac{1}{8}\). Now, use the Pythagorean Identity \(sin^2 \theta +\cos^2 \theta =1\) to find cosine.
\(\begin{aligned}
\left(-\dfrac{1}{8}\right)^{2}+\cos ^{2} \theta &=1 \\
\cos ^{2} \theta &=1-\dfrac{1}{64} \\
\cos ^{2} \theta &=\dfrac{63}{64} \\
\cos \theta &=\pm \dfrac{3 \sqrt{7}}{8} \\
\cos \theta &=-\dfrac{3 \sqrt{7}}{8}
\end{aligned}\)
\(\sec\theta =−\dfrac{8}{3\sqrt{7}}=−\dfrac{8\sqrt{7}}{21}\), \(\tan\theta =\dfrac{1}{3\sqrt{7}}=\dfrac{\sqrt{7}}{ 21}\), and \(\cot\theta =3\sqrt{7}\)
Review
- In which quadrants is the sine value positive? Negative?
- In which quadrants is the cosine value positive? Negative?
- In which quadrants is the tangent value positive? Negative?
Find the values of the other five trigonometric functions of \(\theta \).
- \(\sin\theta =\dfrac{8}{17}\), \(0<\theta <\dfrac{\pi}{2}\)
- \(\cos\theta =−\dfrac{5}{6}\), \(\dfrac{\pi}{2}<\theta <\pi\)
- \(\tan\theta =\dfrac{\sqrt{3}}{4}\), \(0<\theta <\dfrac{\pi}{2}\)
- \(\sec\theta =−\dfrac{41}{9}\), \(\pi <\theta <\dfrac{3\pi}{2}\)
- \(\sin\theta =−\dfrac{11}{14}\), \(\dfrac{3\pi}{2}<\theta <2\pi\)
- \(\cos\theta =\dfrac{\sqrt{2}}{2}\), \(0<\theta <\dfrac{\pi}{2}\)
- \(\cot\theta =\sqrt{5}\), \(\pi <\theta <\dfrac{3\pi}{2}\)
- \(\csc\theta =4\), \(\dfrac{\pi}{2}<\theta <\pi\)
- \(\tan\theta =−\dfrac{7}{10}\), \(\dfrac{3\pi}{2}<\theta <2\pi\)
- Aside from using the identities, how else can you find the values of the other five trigonometric functions?
- Given that \(\cos\theta =\dfrac{6}{11}\) and \(\theta\) is in the \(2^{nd}\) quadrant, what is \(\sin(−\theta )\)?
- Given that \(\tan\theta =−\dfrac{5}{8}\) and \(\theta\) is in the \(4^{th}\) quadrant, what is \(\sec(−\theta )\)?
Answers for Review Problems
To see the Review answers, open this PDF file and look for section 14.7.