For a spherical triangle ABC, pick up incorrect statement

- 1
  
  $\tan \frac{1}{2} (a + b) = \frac{\cos \frac{1}{2} (A - B)}{\cos \frac{1}{2} (A + B)} \tan \frac{1}{2} c$
- 2
  
  $\tan \frac{1}{2} (a - b) = \frac{\sin \frac{1}{2} (A - B)}{\cos \frac{1}{2} (A + B)} \tan \frac{1}{2} c$
- 3
  
  $\tan \frac{1}{2} (A + B) = \frac{\sin \frac{1}{2} (a - b)}{\sin \frac{1}{2} (a + b)} \tan \frac{1}{2} c$
- 4
  
  none of these

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