### Home > PC > Chapter 6 > Lesson 6.3.1 > Problem6-109

6-109.

Double - Angle Formulas
sin(2α) = 2sin(α)cos(α)
cos(2α) = cos2(α) − sin2(α)

Half - Angle Formulas

$\sin\left ( \frac{\theta }{2} \right )=\pm \sqrt{\frac{1-\cos(\theta)}{2}}$

$\cos\left(\frac{\theta }{2} \right )=\pm \sqrt{\frac{1+\cos(\theta)}{2}}$

Draw a unit circle and include a right triangle matching the given conditions.
Use the Pythagorean Theorem to determine the value of the missing leg.