How do you use a half-angle formula to simplify sin -105? Trigonometry Trigonometric Identities and Equations Half-Angle Identities 1 Answer Nghi N. Jun 6, 2015 sin -105 = - sin 105 cos 210 = cos (30 + 180) = - cos 30 = -(sqrt3)/2 2sin^2 105 = 1 - cos 210 = 1 + 0.866 = 1.866 sin^2 105 = 1.866/2 = 0.933 -> sin x = +- 0.97 sin -105 = - 0.97 Answer link Related questions What is the Half-Angle Identities? How do you use the half angle identity to find cos 105? How do you use the half angle identity to find cos 15? How do you use the half angle identity to find sin 105? How do you use the half angle identity to find tan (pi/8)? How do you use half angle identities to solve equations? How do you solve \sin^2 \theta = 2 \sin^2 \frac{\theta}{2} over the interval [0,2pi]? How do you find the exact value for sin105 using the half‐angle identity? How do you find the exact value for cos165 using the half‐angle identity? How do you find the exact value of cos15using the half-angle identity? See all questions in Half-Angle Identities Impact of this question 1826 views around the world You can reuse this answer Creative Commons License