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Trigonometry
Question

Use a sum or difference formula to find the exact value of the following.

$$\cos \frac { 11 \pi } { 12 } \cos \frac { \pi } { 6 } + \sin \frac { 11 \pi } { 12 } \sin \frac { \pi } { 6 }$$

Calculate the trigonometric value

$$- \frac{\sqrt{2}}{2}$$

Solution
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