Learning Resources

Focus E: More Trigonometric Identities

Answers

1.

• cos2a = cos²a - sin²a.
• From the Pythagorean Identities, you know that cos²a = 1 - sin²a.
• Substituting, the result is as follows:
  cos2a = (1 - sin²a) - sin²a = 1 - 2sin²a

2.

• cos2a = cos²a - sin²a.
• From the Pythagorean Identities, you know that sin²a = 1 - cos²a
• Substituting, the result is as follows:
  cos2a = cos²a - (1 - cos²a) = 2cos²a - 1