TRIGONOMETRY: DOUBLE-ANGLE FORMULAS

The half-angle formulas are used to find the trigonometric function values of half of special or familiar angles.

Note that there are two alternate formulas for tangent of half an angle (Formula C). We usually use the
first form since its denominator has only one term instead of the two terms in the second form. In case the
denominator contains a radical number, it is easier to rationalize the denominator when it has only one term
instead of two.

Example 1

Example 2

Try these problems

QUESTIONS

Use a half-angle formula to find each of the following.

1. 




2. 




3. 




4. 

ANSWERS

1. 

   




2. 

   




3. 

   




4.