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3.4.1: Double and Half Angle Formulas

  • Page ID
    4221
  • Sine, cosine, and tangent of angles other than multiples of 30, 45, and 60 degrees.

    You want to find the exact value of \(\tan \dfrac{ 3 \pi}{8}\). How could you find this value without u\sin g a calculator?

    Double Angle and Half Angle Formulas

    In this concept, we will learn how to find the exact values of the trig functions for angles that are half or double of other angles. Here we will introduce the Double-Angle \((2a)\) and Half-Angle \(\left(\dfrac{a}{2}\right)\) Formulas.

    Double-Angle and Half-Angle Formulas

    \(\begin{aligned}
    \cos 2 a &=\cos ^{2} a-\sin ^{2} a & \sin 2 a&=2 \sin a \cos a \\
    &=2 \cos ^{2} a-1  & \tan 2 a&=\dfrac{2 \tan a}{1-\tan ^{2} a} \\
    &=1-\sin ^{2} a  \\
    \sin \dfrac{a}{2} &=\pm \sqrt{\dfrac{1-\cos a}{2}} & \tan \dfrac{a}{2} &=\dfrac{1-\cos a}{\sin a} \\
    \cos \dfrac{a}{2} &=\pm \sqrt{\dfrac{1+\cos a}{2}} & &=\dfrac{\sin a}{1+\cos a}
    \end{aligned}\)

    The signs of \(\sin \dfrac{a}{2}\) and \(\cos \dfrac{a}{2}\) depend on which quadrant \(\dfrac{a}{2}\) lies in. For \(\cos 2a\) and \(\tan \dfrac{a}{2}\) any formula can be used to solve for the exact value.

    Let's find the exact value of \(\cos \dfrac{\pi}{8}\).

    \(\dfrac{\pi}{8}\) is half of \(\dfrac{\pi}{4}\) and in the first quadrant.

    \(\begin{aligned} \cos \left(\dfrac{1}{2} \cdot \dfrac{\pi}{4}\right)&=\sqrt{\dfrac{1+\cos \dfrac{\pi}{4}}{2}}\\&=\sqrt{\dfrac{1+\dfrac{\sqrt{2}}{2}}{2}}\\&=\sqrt{\dfrac{1}{2} \cdot \dfrac{2+\sqrt{2}}{2}} \\&=\dfrac{\sqrt{2+\sqrt{2}}}{2}\end{aligned}\)

    Now, let's find the exact value of \(\sin 2a\) if \(\cos a=−\dfrac{4}{5}\) and \(\dfrac{3 \pi}{2}\leq a<2\pi\).

    To use the sine double-angle formula, we also need to find \(\sin a\), which would be \(\dfrac{3}{5}\) because a is in the \(4^{th}\) quadrant.

    \(\begin{aligned} \sin 2a&=2\sin a\cos a \\ &=2\cdot \dfrac{3}{5}\cdot −\dfrac{4}{5} \\ &=−\dfrac{24}{25}\end{aligned}\)

    Finally, let's find the exact value of \(\tan 2a\) for \(a\) from the previous problem.

    Use \(\tan a=\sin a\cos a=\dfrac{\dfrac{3}{5}}{−\dfrac{4}{5}}=−\dfrac{3}{4}\) to solve for \(\tan 2a\).

    \(\tan 2a=\dfrac{2\cdot −\dfrac{3}{4}}{1−\left(−\dfrac{3}{4}\right)^2}=\dfrac{−\dfrac{3}{2}}{\dfrac{7}{16}}=−\dfrac{3}{2}\cdot \dfrac{16}{7}=−\dfrac{24}{7}\)

    Example \(\PageIndex{1}\)

    Earlier, you were asked to find the value of \(\tan \dfrac{ 3 \pi}{8}\) without a calculator. 

    Solution

    \(\dfrac{ 3 \pi}{8}=\dfrac{1}{2}\cdot \dfrac{3\pi }{4}\) so we can use the formula \(\tan \dfrac{a}{2}=\dfrac{\sin a}{1+\cos a }\) for \(a=\dfrac{3\pi }{4}\)

    \(\begin{aligned} \tan \dfrac{ 3 \pi}{8}&=\dfrac{\sin \dfrac{3\pi }{4}}{1+\cos \dfrac{3\pi }{4}}\\&=\dfrac{\dfrac{\sqrt{2}}{2}}{1+\dfrac{−\sqrt{2}}{2}} \end{aligned}\)

    If we simplify this expression, we get \(\sqrt{2} +1\).

    Example \(\PageIndex{2}\)

    Find the exact value of \(\cos \left(−\dfrac{5 \pi}{8}\right)\).

    Solution

    \(−\dfrac{5 \pi}{8}\) is in the \(3^{rd}\) quadrant.

    \(\begin{aligned} −\dfrac{5 \pi}{8}=\dfrac{1}{2}\left(−\dfrac{5 \pi}{4}\right) &\rightarrow \cos \dfrac{1}{2}\left(−\dfrac{5 \pi}{4}\right)=−\sqrt{\dfrac{1+\cos \left(−\dfrac{5 \pi}{4}\right)}{2}} \\ &=−\dfrac{1−\dfrac{\sqrt{2}}{2}}{2}=\sqrt{\dfrac{1}{2} \cdot \dfrac{2−\sqrt{2}}{2}}=\dfrac{\sqrt{2−\sqrt{2}}}{2} \end{aligned}\)

    Example \(\PageIndex{3}\)

    Given the function \(\cos a=\dfrac{4}{7}\) and \(0\leq a<\dfrac{\pi}{2}\), find \(\sin 2a\).

    Solution

    First, find \(\sin a\). \(4^2+y^2=7^2 \rightarrow y=\sqrt{33}\), so \(\sin a=\dfrac{\sqrt{33}}{7}\)

    \(\sin 2a=2\cdot \dfrac{\sqrt{33}}{7} \cdot \dfrac{4}{7}=\dfrac{8\sqrt{33}}{49}\)

    Example \(\PageIndex{4}\)

    Given the function \(\cos a=\dfrac{4}{7}\) and \(0\leq a<\dfrac{\pi}{2}\), find \(\tan \dfrac{a}{2}\).

    Solution

    You can use either \(\tan \dfrac{a}{2}\) formula.

    \(\tan \dfrac{a}{2}=\dfrac{1−\dfrac{4}{7}}{\dfrac{\sqrt{33}}{7}}=\dfrac{3}{7} \cdot \dfrac{7}{\sqrt{33}}=\dfrac{3}{\sqrt{33}}=\dfrac{\sqrt{33}}{11}\)

     

    Review

    Find the exact value of the following angles.

    1. \(\sin 105^{\circ}\)
    2. \(\tan \dfrac{\pi}{8}\)
    3. \(\cos \dfrac{ 5 \pi}{12}\)
    4. \(\cos 165^{\circ}\)
    5. \(\sin \dfrac{ 3 \pi}{8}\)
    6. \(\tan \left(−\dfrac{ \pi}{12}\right)\)
    7. \(\sin \dfrac{11 \pi}{8}\)
    8. \(\cos \dfrac{19 \pi}{12}\)

    The \(\cos a=\dfrac{5}{13}\) and \(\dfrac{3 \pi}{2}\leq a<2\pi \). Find:

    1. \(\sin 2a\)
    2. \(\cos \dfrac{a}{2}\)
    3. \(\tan \dfrac{a}{2}\)
    4. \(\cos 2a\)

    The \(\sin a=\dfrac{8}{11}\) and \(\dfrac{\pi }{2} \leq a<\pi \). Find:

    1. \(\tan 2a\)
    2. \(\sin \dfrac{a}{2}\)
    3. \(\cos \dfrac{a}{2}\)
    4. \(\sin 2a\)

    Answers for Review Problems

    To see the Review answers, open this PDF file and look for section 14.15. 

    Additional Resources

    Interactive Element

     

    Practice: Double and Half Angle Formulas

     

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