Review the proof of the identity cos(π − A) = −cosA.cos(π − A)Step 1: = cosπcosA − sinAsinπStep 2: = (−1)(cosA) + (sinA)(0)Step 3: = −1cosA + (sinA)(0)Step 4: = −cosA + 0Step 5: = −cosA

The difference identity is cos(x − y) = cos x cos y + sin x sin y. Therefore, cos(π − A) should be written as cos π cos A + sin π sin A. Step 1 uses a minus sign instead of a plus sign, so the first error is step 1, choice A.

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