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

6-109.

If cos *x* = −*x* ∈* *[*π*, *x*, cos 2*x*, sin (*x* =

Double - Angle Formulas

sin(2*α*) = 2sin(*α*)cos(*α*)

cos(2*α*) = cos^{2}(*α*) − sin^{2}(*α*)

Half - Angle Formulas

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.