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# numpy.arccos

`numpy.``arccos`(x[, out])

  Trigonometric inverse cosine, element-wise.

  The inverse of cos so that, if `y = cos(x)`, then `x = arccos(y)`.

  Parameters: | x : array_like x-coordinate on the unit circle. For real arguments, the domain is [-1, 1].
  Returns: | angle : ndarray The angle of the ray intersecting the unit circle at the given x-coordinate in radians [0, pi]. If x is a scalar then a scalar is returned, otherwise an array of the same shape as x is returned.

  See also

  `cos`, `arctan`, `arcsin`

  Notes

  arccos is a multivalued function: for each x there are infinitely many numbers z such that cos(z) = x. The convention is to return the angle z whose real part lies in [0, pi].

  For real-valued input data types, arccos always returns real output. For each value that cannot be expressed as a real number or infinity, it yields `nan` and sets the invalid floating point error flag.

  For complex-valued input, arccos is a complex analytical function that has branch cuts [-inf, -1] and [1, inf] and is continuous from above on the former and from below on the latter.

  The inverse cos is also known as acos or cos^-1.

  References

  [3] | M. Abramowitz and I.A. Stegun, “Handbook of Mathematical Functions”, 10th printing, 1964, pp. 79. http://www.math.sfu.ca/~cbm/aands/

  [3] | Wikipedia, “Inverse trigonometric function”, http://en.wikipedia.org/wiki/Arccos

  Examples

  We expect the arccos of 1 to be 0, and of -1 to be pi:

      >>> np.arccos([1, -1])
      array([ 0.        ,  3.14159265])

  Plot arccos:

      >>> import matplotlib.pyplot as plt
      >>> x = np.linspace(-1, 1, num=100)
      >>> plt.plot(x, np.arccos(x))
      >>> plt.axis('tight')
      >>> plt.show()

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