How would you apply the Cosine Angle Difference Identity to find the exact value of cos 75o?

Write 75° as 120° − 45°, so choice B lets you use the cosine difference identity: cos(a − b) = cos a cos b + sin a sin b. Substituting gives cos 75° = (−1/2)(√2/2) + (√3/2)(√2/2) = (√6 − √2)/4. These angles have familiar exact trigonometric values, making this a useful way to find the exact value.

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