Given that tanθ=−7/6

and π/2<θ<π, find the exact values of the trigonometric functions.



sin2θ=

csc2θ=

cos2θ=

sec2θ=

tan2θ=

cot2θ=

1 answer

Given that \(\tan \theta = -\frac{7}{6}\) and \(\frac{\pi}{2} < \theta < \pi\), we can determine the values of the trigonometric functions using the properties of the triangle and identities.

Step 1: Determine values of \(\sin \theta\) and \(\cos \theta\)

Since \(\tan \theta = \frac{\sin \theta}{\cos \theta}\), we can establish the following:

\[ \sin \theta = -7k \quad \text{and} \quad \cos \theta = 6k \]

where \(k\) is some positive constant. To maintain consistency with the given interval \(\frac{\pi}{2} < \theta < \pi\) which corresponds to Quadrant II, \(\sin \theta\) is positive and \(\cos \theta\) is negative.

Thus, let:

\[ \sin \theta = 7k \quad \text{and} \quad \cos \theta = -6k \]

To find \(k\), we must apply the Pythagorean identity:

\[ \sin^2 \theta + \cos^2 \theta = 1 \]

Substituting the values of \(\sin \theta\) and \(\cos \theta\):

\[ (7k)^2 + (-6k)^2 = 1 \]

Calculating this gives:

\[ 49k^2 + 36k^2 = 1 \]

\[ 85k^2 = 1 \implies k^2 = \frac{1}{85} \implies k = \frac{1}{\sqrt{85}} \]

Now plug \(k\) back into our expressions for \(\sin \theta\) and \(\cos \theta\):

\[ \sin \theta = 7 \cdot \frac{1}{\sqrt{85}} = \frac{7}{\sqrt{85}} \]

\[ \cos \theta = -6 \cdot \frac{1}{\sqrt{85}} = -\frac{6}{\sqrt{85}} \]

Step 2: Calculate \(\sin 2\theta\), \(\cos 2\theta\), and \(\tan 2\theta\)

Now, we can use the double angle formulas:

  1. For \(\sin 2\theta\):

\[ \sin(2\theta) = 2 \sin \theta \cos \theta \]

Substituting the values:

\[ \sin(2\theta) = 2 \left(\frac{7}{\sqrt{85}}\right) \left(-\frac{6}{\sqrt{85}}\right) = -\frac{84}{85} \]

So, \(\sin 2\theta = -\frac{84}{85}\).

  1. For \(\cos 2\theta\):

\[ \cos(2\theta) = \cos^2 \theta - \sin^2 \theta \]

Calculating \( \cos^2 \theta \) and \( \sin^2 \theta \):

\[ \cos^2 \theta = \left(-\frac{6}{\sqrt{85}}\right)^2 = \frac{36}{85} \]

\[ \sin^2 \theta = \left(\frac{7}{\sqrt{85}}\right)^2 = \frac{49}{85} \]

Thus,

\[ \cos(2\theta) = \frac{36}{85} - \frac{49}{85} = -\frac{13}{85} \]

So, \(\cos 2\theta = -\frac{13}{85}\).

  1. For \(\tan 2\theta\):

Using the identity \(\tan(2\theta) = \frac{\sin(2\theta)}{\cos(2\theta)}\):

\[ \tan(2\theta) = \frac{-\frac{84}{85}}{-\frac{13}{85}} = \frac{84}{13} \]

Step 3: Calculate \( \csc 2\theta\), \( \sec 2\theta\), and \( \cot 2\theta\)

Now we find the other trigonometric functions:

  1. For \(\csc 2\theta\):

\(\csc 2\theta = \frac{1}{\sin 2\theta} = \frac{1}{-\frac{84}{85}} = -\frac{85}{84}\).

  1. For \(\sec 2\theta\):

\(\sec 2\theta = \frac{1}{\cos 2\theta} = \frac{1}{-\frac{13}{85}} = -\frac{85}{13}\).

  1. For \(\cot 2\theta\):

\(\cot 2\theta = \frac{1}{\tan 2\theta} = \frac{13}{84}\).

Summary of results:

  • \(\sin 2\theta = -\frac{84}{85}\)
  • \(\csc 2\theta = -\frac{85}{84}\)
  • \(\cos 2\theta = -\frac{13}{85}\)
  • \(\sec 2\theta = -\frac{85}{13}\)
  • \(\tan 2\theta = \frac{84}{13}\)
  • \(\cot 2\theta = \frac{13}{84}\)