Q. Use the reference angle to find the exact value of the given expression.cos(67π)
Identify Reference Angle: Identify the reference angle for (7π)/6. Since (7π)/6 is in the third quadrant (more than π but less than 3π/2), the reference angle is π−(7π)/6=(6π)/6−(7π)/6=−(π)/6.
Determine Cosine: Determine the cosine of the reference angle.The cosine of (π)/6 is 3/2. Since the cosine is positive in the first quadrant and we are looking for the cosine of the negative of this angle, it remains positive: cos(−(π)/6)=cos((π)/6)=3/2.
Apply Function Behavior: Apply the cosine function's behavior in different quadrants.In the third quadrant, where (7π)/6 lies, the cosine function is negative. Therefore, we must take the negative of the reference angle's cosine value.
Combine Results: Combine the results to find the exact value.The exact value of cos(67π) is −3/2.