Split into two angles where the values of the six trigonometric functions are known.

Separate negation.

Apply the difference of angles identity.

The exact value of is .

The exact value of is .

The exact value of is .

The exact value of is .

Simplify each term.

Multiply and .

Move to the left of .

Multiply and .

Move to the left of .

Combine the numerators over the common denominator.

The result can be shown in multiple forms.

Exact Form:

Decimal Form:

Find the Exact Value cos(15 degrees )