Use the identities for the cosine of a sum or difference to write each expression as a trigonometric function of θ alone.

cos(90° + θ)

Find the exact value of each expression. (Do not use a calculator.)

cos(-15°)

Find the exact value of each expression. (Do not use a calculator.)

cos 105° (Hint: 105° = 60° + 45°)

Find the exact value of each expression. (Do not use a calculator.)

cos π/12