Geometry

Question Analysis  

The main focus is using the co - function identity for complementary angles, along with angle - reduction techniques, to represent in terms of sine. Since is an obtuse angle,  we first need to transform it into an equivalent acute - related angle before applying the co - function identity.

Key Concept Explanation  

Co - function identity: applies to acute angles. For non - acute angles, we can use angle - reduction formulas. For an angle in the second quadrant (<<), . After reducing the angle to an acute one, we can apply the co - function identity. Also, recall that the cosine function is negative in the second quadrant.

Step-by-Step Solution  

1. Rewrite using the angle - reduction formula:

Since , then .

2. Apply the co - function identity:

Using the co - function identity

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