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3.6: Half-Angle Identities

Difficulty Level: At Grade Created by: CK-12

Learning Objectives

  • Apply the half angle identities to expressions, equations and other identities.
  • Use the half angle identities to find the exact value of trigonometric functions for certain angles.

Just as there are double angle identities, there are also half angle identities. For example: can be found in terms of the angle . Recall that and are the same thing and will be used interchangeably throughout this section.

Deriving the Half Angle Formulas

In the previous lesson, one of the formulas that was derived for the cosine of a double angle is: . Set , so the equation above becomes .

Solving this for , we get:

if is located in either the first or second quadrant.

if is located in the third or fourth quadrant.

Example 1: Determine the exact value of .

Solution: Using the half angle identity, , and is located in the first quadrant. Therefore, .

Plugging this into a calculator, . Using the sine function on your calculator will validate that this answer is correct.

Example 2: Use the half angle identity to find exact value of

Solution: since , use the half angle formula for sine, where . In this example, the angle is a second quadrant angle, and the sin of a second quadrant angle is positive.

One of the other formulas that was derived for the cosine of a double angle is:

. Set , so the equation becomes . Solving this for , we get:

if is located in either the first or fourth quadrant.

if is located in either the second or fourth quadrant.

Example 3: Given that the , and that is a fourth quadrant angle, find

Solution: Because is in the fourth quadrant, the half angle would be in the second quadrant, making the cosine of the half angle negative.

Example 4: Use the half angle formula for the cosine function to prove that the following expression is an identity:

Solution: Use the formula and substitute it on the left-hand side of the expression.

The half angle identity for the tangent function begins with the reciprocal identity for tangent.

The half angle formulas for sine and cosine are then substituted into the identity.

At this point, you can multiply by either or . We will show both, because they produce different answers.

So, the two half angle identities for tangent are and .

Example 5: Use the half-angle identity for tangent to determine an exact value for .

Solution:

Example 6: Prove the following identity:

Solution: Substitute the double angle formulas for and .

Solving Trigonometric Equations Using Half Angle Formulas

Example 7: Solve the trigonometric equation over the interval .

Solution:

Then or , which is .

.

Points to Consider

  • Can you derive a third or fourth angle formula?
  • How do and differ? Is there a formula for ?

Review Questions

  1. Find the exact value of:
  2. If and is in Quad II, find
  3. Prove the identity:
  4. Verify the identity:
  5. Prove that
  6. If , find
  7. Solve for
  8. Solve for
  9. Solve the trigonometric equation such that .
  10. Prove .

Review Answers

  1. If , then by the Pythagorean Theorem the third side is 24. Because is in the second quadrant, .
  2. Step 1: Change right side into sine and cosine. Step 2: At the last step above, we have simplified the right side as much as possible, now we simplify the left side, using the half angle formula.
  3. Step 1: change cotangent to cosine over sine, then cross-multiply.
  4. First, we need to find the third side. Using the Pythagorean Theorem, we find that the final side is . Using this information, we find that . Plugging this into the half angle formula, we get:
  5. To solve , first we need to isolate cosine, then use the half angle formula. when
  6. To solve , first isolate tangent, then use the half angle formula. Using your graphing calculator, when
  7. This is the two formulas for . Cross-multiply.

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CK.MAT.ENG.SE.2.Trigonometry.3.6

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