Sum Difference Identities

Use a sum or difference identity to find the EXACT value:

$$\sin\left(15\degree\right)$$  

Use a sum or difference identity to find the EXACT value:
$$\cos\left(75\degree\right)$$  

Write the expression as the sine or cosine of a single angle:

  $$\sin42\degree\cos17\degree-\cos42\degree\sin17\degree$$  

Write the expression as the sine or cosine of a single angle:

$\cos\left(7y\right)\cos\left(3y\right)-\sin\left(7y\right)\sin\left(3y\right)$

Write the following expression as the sine, cosine, or tangent of an angle.
cos(175)cos(55)+sin(175)sin(55)

Find the exact value.

Please choose the correct sum and difference formula:
cos (α-β) =

Use sum or difference angles identity to find the exact value for $$\cos105^o$$  

Use sum or difference angles identity to find the exact value for       $$\sin\ \left(-15^o\right)$$