Question

Don't understand this concept! Please help!!!

Answer 1

Okay, so my question concerns the Half-Angle Identities of sin, cos, and tan. There's a plus/minus sign in front of the sqrt. How do I know when to use what? For an example, if the answer to cos (\[\alpha\]/2) is \[\frac {2\sqrt{5}}{5}\] HOW Do I know what quadrant it is in? My teacher said it was in teh 2nd quadrant, so it is negative. but I don't understand the p/m sign!! Please help!!!!!

Answer 2

@hoblos @dpalnc anyone?