Geometry

Question Analysis  

The main focus is to use the co - function identity for complementary angles, but with the consideration that we first need to find an equivalent acute - related angle, since the co - function identity is based on acute complementary angles. For an obtuse angle like , we'll use angle - reduction techniques.

Key Concept Explanation  

Co - function identity: holds for acute angles. For non - acute angles, we can use the property that . Then we can find the complementary angle of the resulting acute angle. Also, recall that the sine function is positive in the second quadrant (< < ) where lies.

Step-by-Step Solution  

1. Rewrite using the angle - reduction formula:

Since , for ,

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