Geometry

Question Analysis  

The main focus is to use the Pythagorean identity  to find  when  and  is in Quadrant III, where  is negative.  

 

Key Concept Explanation  

Pythagorean identity: Relates  and  via .  

Quadrant signs: In QIII, both  and  are negative.  

 

Step-by-Step Solution  

1. Substitute  into the identity:  

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