1)

Question

Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number.

cos 12° cos 18° − sin 12° sin 18°

And Find its exact value.

2)
Use an Addition or Subtraction Formula to write the expression as a trigonometric function of one number.

sin 4pi/5 cos 7pi/15 - cos 4pi/5 sin 7pi/15

And find its exact value.

3)

Write the trigonometric expression in terms of sine and cosine, and then simplify.

tan θ / cos θ − sec θ

Answer 1

1) that would be cos(12°+18°) which I'm sure you know

2) sin(4π/5-7π/5)

3)
well,

tan/cos = sin/cos^2 and sec = 1/cos

Answer 2

Thank you so much but are those the expressions or exact answers for the first two problems?

Answer 3

they are expressions. You surely know the exact value of cos(30°)

as for sin(-3π/5) you would have to construct a pentagon and figure out some side lengths. figure out sin 36° and then sin 72° and then the sine of 108°.