cos ( -pi/12) <- Find the exact values ​​using the addition or subtraction formulas

Question

Find the exact values ​​using the addition or subtraction formulas.

Please show the process i do not understand :(

Answer 1

Cosine is an odd function, so cos(-π/12) = -cos(π/12)

π/12 = (π/12)(180°/π) = 15°

So, -cos(π/12) = -cos(45° - 30°) = -[cos45°cos30° + sin45°sin30°] = -[(√2/2)(√3/2) + (√2/2)(1/2)} = (-√6-√2)/4