Question

How do we know if our half angle formula is positive or negative?

Answer 1

Automatic according to the angles u are using.

Answer 2

so... should this be correct?
\[\sin (x/2) = -\sqrt{(1-\cos x)/2}\]
if\[\cos x = 5/13\] and \[\sin x < 0\]

Answer 3

Ok, u square rooted the double angle formula and chose the negative is that right?

Answer 4

yes but is negative correct? or should it be positive?

Answer 5

given the conditions stated above

Answer 6

I would not choose either, I would do the calculations first ie plus and minus, then see what I had.