6 We can also derive formulas for an angle divided by 2. Half-Angle FormulasAs stated it is NOT both + and - but you must figure out where the terminal side of the angle is and put on the appropriate sign for that quadrant.
8 We could find sin 15° using the half angle formula. 30°30°Since 15° is half of 30° we could use this formula if = 30°15° is in first quadrant and sine is positive there so we want the +
9 Let's draw a picture.54-3Use triangle to find values.
10 If is in quadrant II then half would be in quadrant I where sine is positive 54-3Use triangle to find cosine value.
11 AcknowledgementI wish to thank Shawna Haider from Salt Lake Community College, Utah USA for her hard work in creating this PowerPoint.Shawna has kindly given permission for this resource to be downloaded from and for it to be modified to suit the Western Australian Mathematics Curriculum.Stephen CorcoranHead of MathematicsSt Stephen’s School – Carramar