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Can \( \sin z \) exceed one in modulus?

  • A.Yes, it is unbounded on the imaginary axis
  • B.No, it is bounded by one as in the real case
  • C.Yes, but only within the unit disc
  • D.No, its modulus never exceeds \( \pi \)

Answer: A

On the imaginary axis, with \( z = iy \),

\[ \sin(iy) \;=\; i\sinh y, \]

whose modulus grows without bound as \( y \) does. The familiar bound of one holds only on the real axis; in the complex plane the trigonometric and hyperbolic functions are the same family seen in two directions

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