Find the Exact Value cos((11pi)/4)

Problem

cos((11*π)/4)

Solution

1. Find the coterminal angle by subtracting multiples of 2*π until the angle is within the interval [0,2*π)

2*π=(8*π)/4

(11*π)/4−(8*π)/4=(3*π)/4

2. Identify the quadrant for the angle (3*π)/4 Since π/2<(3*π)/4<π the angle is in Quadrant II.

3. Determine the reference angle for (3*π)/4 in Quadrant II.

π−(3*π)/4=π/4

4. Apply the cosine sign for Quadrant II. In Quadrant II, the cosine function is negative.

cos((3*π)/4)=−cos(π/4)

5. Substitute the exact value for cos(π/4) which is √(,2)/2

−cos(π/4)=−√(,2)/2

Final Answer

cos((11*π)/4)=−√(,2)/2

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