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9-86.
  1. Challenge: Given f(x) = tan x. Homework Help ✎

    1. Use the fact that sin(x +y) = sin x cos y + cos x sin y
      and cos(x + y) = cos x cos y − sin x sin y to show that
      .

    2. At this point, can you find ? Explain.

    3. Find the resulting equation when multiplying by .

    4. It is known that . Use this fact and your answer to part (c) to show that when
      for f(x) = tan x is applied, the result is sec2x.

No, you would have division by 0.