### Home > PC3 > Chapter 8 > Lesson 8.3.4 > Problem 8-137

8-137.

If cos(*x*) = –*x*) < 0, calculate exact values of sin(2*x*), cos(2*x*), sin

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.