Tangent half angle formula

Question

So we start with the following tangent half angle formula: $$ \tan\left(\frac \theta2\right) = \pm\sqrt{\frac {1 - \cos \theta}{1 + \cos \theta}} $$

If I do some algebraic manipulation I end up with the following below: $$ \tan \left(\frac \theta2\right)= \pm\frac {1 - \cos \theta} {\sin \theta}$$

Now according to Michael Corral's Trigonometry the minus sign is not possible so I only end up with:

$$ \tan \left(\frac \theta2\right)= \frac {1 - \cos \theta} {\sin \theta} $$

Can you please explain why that is true?

When using trig functions make sure you do \tan instead of tan — K Split X, Apr 19 '17 at 21:16
@Omicron. Realize that $tan\theta/2$ is negative for exactly the same value of $\theta$ as when the sine is negative. So that's how the plus/minus signs are taken care off. You do not need the absolute values. Consider $\theta$ in Quadrant 3 and 4 — imranfat, Apr 19 '17 at 21:38

score 3 · Answer 1 · answered Apr 19 '17 at 22:11

The fussiness over $\pm$ arises because $\tan \frac\theta2$ can be either positive or negative, whereas the square root is always considered positive. Your algebraic manipulation of $\sqrt{\frac{1-\cos\theta}{1+\cos\theta}}$ is OK except in the final step: the result should be $\left|\frac{1-\cos\theta}{\sin\theta}\right|$, as in @haqnatural's derivation, which leads to $$ \tan\frac\theta2 = \pm\left|\frac{1-\cos\theta}{\sin\theta}\right|.\tag1 $$ Equation (1) is true, but doesn't get us any closer to resolving the question of which sign to choose! One way to decide this is to consider different ranges of $\theta$, as in @imranfat's answer.

If this is not satisfying, you can try an alternative derivation that doesn't fuss with plus and minus signs: $$ \tan\frac\theta2=\frac{\sin\frac\theta2}{\cos\frac\theta2}=\frac{2\sin\frac\theta2\sin\frac\theta2}{2\sin\frac\theta2\cos\frac\theta2}=\frac{1-\cos\theta}{\sin\theta}, $$ using the double-angle identities $$ \cos2t=1-2\sin^2t $$ and $$\sin2t = 2\sin t\cos t.$$

score 1 · Answer 2 · answered Apr 19 '17 at 21:52

One way to tackle this problem is to look at the sign outcome of $tan\theta/2$. This happens if $\theta$ is on interval $(\pi,2\pi)$. In other words, when $\theta$ (not $\theta/2$!!) is in Quadrant 3 or 4, $tan\theta/2<0$. Now in $\frac{1-cos\theta}{sin\theta}$, the numerator is positive, so consider the denominator. The sine graph produces negative values when $\theta$ is in Q3 or Q4 and so the outcome is negative.Similarly for angles given in Quadrant 1 and 2, the sine outcome is positive. Now for the expression that contains a square root, the author puts the plus/minus in front but the reader need to determine the quadrant in question and then chose the correct sign. Otherwise, the correct identity is $tan\theta/2=\frac{1-cos\theta}{sin\theta}$ without the absolute values.

Tangent half angle formula

2 Answers2