Add to .

The resulting angle of is coterminal with but isn’t positive. Repeat the step.

Add to .

The resulting angle of is coterminal with but isn’t positive. Repeat the step.

Add to .

The resulting angle of is coterminal with but isn’t positive. Repeat the step.

Add to .

The resulting angle of is coterminal with but isn’t positive. Repeat the step.

Add to .

The resulting angle of is positive and coterminal with .

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 .

Multiply by .

Find the Exact Value cos(-9pi)