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 third quadrant.

The exact value of is .

Multiply by .

Evaluate cos(-90 degrees )

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# Evaluate cos(-90 degrees )

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 third quadrant.

The exact value of is .

Multiply by .

Evaluate cos(-90 degrees )