The angle is an angle where the values of the six trigonometric functions are known. Because this is the case, add to keep the value the same.

Use the sum formula for cosine to simplify the expression. The formula states that .

The exact value of is .

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant. Make the expression negative because cosine is negative in the second quadrant.

The exact value of is .

The exact value of is .

Apply the reference angle by finding the angle with equivalent trig values in the first quadrant.

The exact value of is .

Multiply by .

Multiply by .

Add and .

The result can be shown in multiple forms.

Exact Form:

Decimal Form:

Expand Using Sum/Difference Formulas cos(135)