Best answer

Theta/2 is between (pi)/4 and (pi)/2 (45 and 90 degrees). This puts all trig functions in the first quadrant where they are all positive. Cosine is negative in quadrants 2 and 3 only. (Quadrant rule for quadrants 1 to 4 is All Silver Tea Cups, or All, Sine, Tangent, Cosine positive rule.) So cos(theta/2)<0 is false when (pi)/2<theta<(pi).

- All categories
- Pre-Algebra Answers 12,260
- Algebra 1 Answers 25,165
- Algebra 2 Answers 10,398
- Geometry Answers 5,143
- Trigonometry Answers 2,632
- Calculus Answers 5,925
- Statistics Answers 3,008
- Word Problem Answers 9,974
- Other Math Topics 6,521

81,026 questions

85,067 answers

2,134 comments

68,484 users